Predicting the airline industry is impossible. The new hire brasilia class two weeks before mine got a line right out of training and I was told to expect the same. I ended up spending 8 months on reserve in the most junior base. The classes right after mine spent over a year on reserve. In the past people have upgraded to the brasilia in under a year.

If movement really picks up you can expect a short amount of time on reserve on the brasilia. Probably not more than a few months. You should always be prepared to spend years on reserve if things slow down right after you're hired though.