Hi.! When you buy the tickets to enter Machu Picchu the office automatically checks the availability in order to sell them to you. This is basically to have a control over the flow of people, a lot of them want to get in early in the morning, but this place cannot receive hundreds of tourist just in the morning and then zero people in the afternoon. My ticket was scheduled on 12:00, I arrived the main gate at 11.30 am, and I was very lucky to get in 30 minutes before (it was an exception ) The people in the main gate scan your ticket barcode and you must show your passport. Don't hesitate if you are late for instance 10 or 15 minutes, you will be admitted. What you cannot do for instance is going there at 08:00 am with the hope to get in early if your ticket was sold for 12:00m. The most import thing here is > 1. buying your tickets in Cusco, so you can be sure you will enter Machu Picchu, you can also buy them in Aguas Calientes (also named Machu Picchu Town) but this is your last chance to get tickets. >2. Planning very well the way you will be traveling to Aguas Calientes from Cusco ( you can check my review on Machu Picchu for detailed further information) If you need more help, I will be glad to answer your questions. Best regards. B@rbie.- 4 hours to visit the park is perfect time to stay in Machu Picchu, enjoy the place, take pictures, etc.