Ten to fourteen days is the sweet spot for a first trip to Peru: enough for Lima, the Cusco region with Machu Picchu, and one more area. Five days is the practical minimum for Cusco and Machu Picchu alone, and seven is more comfortable. A month is worth it only if you want to add the Amazon, the north or long treks.
Count nights in each place, not days in the country, and remember that internal flights and altitude eat into the schedule.
Start from the places, not from the calendar
The most useful way to answer the question is to decide which places you want and add up what each really needs. Peru is not a single compact destination: the coast, the Andes and the rainforest are separate worlds, joined by flights or long bus rides.
Think in nights per base. A base needs at least two nights to be worth the transfer; one night is a stopover. The first day after a flight is mostly spent arriving, and the last day before an international flight is mostly spent leaving, so a trip of seven days has fewer than seven days of sightseeing. Why five days is not really five shows the arithmetic in detail.
What each region needs
Lima rewards two nights: one full day for the districts, museums and food, and a second to recover from the flight. It can be cut to a single night if you are only passing through. For how to use a short stay, see Lima.
Cusco and the Sacred Valley are the heart of most trips. Plan at least four to five nights for Cusco, the valley and Machu Picchu together, with extra time if you intend to trek. Machu Picchu itself is a half-day visit, but getting there and back takes the rest of two days; see how many days Machu Picchu needs. Arequipa and the Colca Canyon need three to four nights, since the canyon is a long way from the city.
The rest of the map
Lake Titicaca and Puno are worth two nights, ideally after you have adapted to altitude. The coast around Paracas and Huacachina is a two-night add-on. The Amazon, whether Puerto Maldonado or Iquitos, needs at least three nights once you subtract the flights. The north is a trip of its own, around ten days.
The Lake Titicaca and the Paracas show what each of the add-ons offers. If you are unsure whether a place is worth the nights, the comparisons weigh it against the alternatives, and too many cities gives a rule for counting bases against nights.
Typical lengths and what they buy
Five days: Lima briefly, then Cusco, the valley and Machu Picchu. It is tight but feasible. Seven days: the same region with room to breathe, or a lighter first day at altitude. Ten days: Lima, Cusco region and one more area, usually Arequipa and the Colca Canyon.
Fourteen days: the classic first-time route, with coast, south and Cusco. Twenty-one days: add Lake Titicaca and a rainforest lodge. Thirty days: a slow trip with rest days, the north or the Cordillera Blanca. Each of these is designed separately rather than stretched: the 5-day, 7-day, 10-day, 14-day and 21-day routes. For all of them side by side, see all routes.
Add buffer, not more sights
If you have extra days, the best use is rarely another city. Keep one spare day before an international flight, so a delayed domestic flight or a road blockade cannot cost you the connection. Add a rest day after a trek. Spend a third night somewhere you liked.
Flights in the mountains are affected by weather, and long bus journeys can be delayed by roadworks or landslides in the rainy season. Spare time absorbs these problems. A tight route does not. Time lost in transit explains how much time a flight or bus really costs, and what to do when weather blocks your route shows why a spare day is cheap insurance.
Who needs more days, and who needs fewer
Some travellers need more days than the table suggests. People who move slowly, who travel with children or older relatives, who want to trek, or who want to learn something about a place rather than see it, should add a night to every base. People who are happy with early starts and long days, who have visited high places before and who want a first taste of Peru, can often manage with the shorter figures.
Time of year matters too. In the rainy season, more spare days are needed because delays are more likely. In the dry season, the limit is not weather but availability: Machu Picchu entry, trains and treks can sell out, and a longer trip can make it easier to find the dates that are free. Choose the length last, after the places and the season.
A simple way to decide
Write the list of places you want, give each its minimum nights from the figures above, and add a night for each flight into the mountains and one buffer night at the end. The total is your minimum. If it exceeds the time you have, remove the place with the longest transfer, not the one with the shortest stay.
If the total is under the time you have, spend the surplus on rest days and a slower pace rather than on a new region. For a first trip, ten to fourteen days is the usual answer, and the 10-day route and 14-day route are good starting points. For first-time travellers, the remaining decisions are covered in detail.
Frequently asked questions
Is one week enough for Peru?
It is enough for one region, normally Cusco, the Sacred Valley and Machu Picchu, with a night or two in Lima. It is not enough for the whole country.
Is two weeks too long for a first trip?
No. Two weeks is the most common length for a first visit and allows three regions at a comfortable pace.
Can I do Machu Picchu in a day?
It is possible from Cusco by train, but it is a very long day. A night in the valley or in Aguas Calientes makes the visit calmer.
How many days do I need for the Inca Trail?
The trail itself is four days, but add at least three days before it for acclimatisation, and one day after it. A full trek trip is therefore about ten days at the least.
What is the minimum for Lima, Cusco and Arequipa?
Nine days is a practical minimum: two in Lima, five in the Cusco region and two in Arequipa. With less, you spend most of the time in airports.
Does the time of year change how many days I need?
A little. In the rainy season you need a spare day or two, because delays are more likely. In the dry season the limit is availability of tickets and trains.
Is a month better than two weeks?
Only if you want to add the north, the Amazon or a long trek. A month of the same places gives more rest, not more sights.