Stig-
I've been heli-skiing in the Cariboos and Monashees (just NW of Revelstoke) for over 30 years.
Both of the prior replies are dead-on accurate.
Please ask yourself how comfortable you are skiing in trees as this dictates your timing. Personally, I like tree skiing as the trees protect the snow so it tends to be deeper.
The earlier you go, the higher the chance you will be skiing in the trees. That said, last December we were on the glacier in the sun and the last week in February it snowed every day and we spent the whole week in the trees. It's weather.
I hope this helps.