I haven't gained all that much on Lexapro, but when I gained a ton on Paxil, it took awhile for it to go, and then it kind of went quickly. Gained some back when I went on Lexapro, but as I say, not nearly as much as on Paxil.
As long as it took to lose it under any circumsance. You have to passionately persue it mentally and physically. Consider it a challenge and conquer the POSSIBLE!