Depends on a few things - like how big the window is. You probably should work out the sq meterage and check the coverage on the paint tin when buying.
I painted my dcs's rooms recently - needed just one and a bit large (2.5l?) tins for the ds's - decided that I must then have enough in one 2.5l tin (left over from another job) to do just three walls of dd's room (did the other wall a different colour). I forgot that the ds's room has a huge window taking up most of one wall - barely had enough for dd's!