When (the number of days prior to departure) starts pre-sale tickets for the bus?
Under current legislation, pre-sale tickets open no later than 15 days before the departure schedule, many bus stations open for sale before 30-45 days.
The last date of sale on a specific bus station you can see here: http://ticket.bus.com.ua/order
The deadline for the sale at a particular bus station is not necessarily applicable to all flights. Some (or most) flights can be opened for sale at other times.