In Australia you can get guests immediately. I currently have just 1 hostel (120 beds) but use to have 300 beds and it was much they same. The majority of my bookings come in the last 3 days. Other countries and cities will have completley different stats. 30-50% of my guests will extend their booking and stay longer.
If you knew exactly which day you planned to open your business this would enable you to have availability on your booking channels potentially months in advance... Booking channels being Booking.com, Agoda, Hostelworld, Hostelbookers, Expedia and Airbnb for example.
It will depend if you open in the high or low season of your area. If possible time it to open for the high season to optimise your cashflow.
You could offer heavily discounted prices to fill your hostel, create a good atmosphere (nobody wants to go in an empty hostel) and then increase the prices, some of the guests will extend and then you will have new check-ins.