The actual transaction will take no more than a minute or two (it's all on their system, so they just type "Warschau" or the appropriate code and out comes a ticket - it's no different from buying an internal ticket). However, as the previous poster said, you should allow plenty of time for queuing. If you want to buy the ticket on the day of departure plan to be there at least an hour before, but if at all possible go a day or so earlier.

Friday and Sunday are busy days for travelling so a seat reservation would be highly advisable