Pre booking for burj Khalifa is must. Otherwise there are no tickets available for the peak time. You can stay as long as you want at the top. It is best to be there an hour before sunset so we can enjoy every view. I.E before sunset, at the time of sunset and after that. Each view has its own charm. I booked a slot of 4;30pm and was there till 8pm. I made the most of it. Sunsets are my favorite and so it was from burj khalifa. They do offer an upgrade from 124th floor to 148th floor for more money but it hardly makes any difference. Dubai from the top is one time view.