Depends on the resort, Pamporovo normally end of March but because easter is in April this year they have extended it madness!!! Bansko and Boro normally into mid April and Bansko was skiin into May the other year.
It may be a curt answer, but what you say is true, they will keep the slopes open as long as there is snow, but most UK tour operators cease on the last week of March. Skiing has continued in recent years though into May but I wouldn't risk that