RE: "how often", I think the obvious answer would be "as often as needed". In the case of Alex Brightman, my guess is that the weight gain was initially to make him look specifically more like Jack Black, but I don't believe the weight is essential to the character, and was kind of a cop-out to begin with. The fact that he can still be accepted as the character now, without the extra weight might bear that out.
I think there are plenty of costuming and make-up tricks that can make an actor appear heavier or thinner without having to gain or lose weight. I think, too that it's more often the case that it only really matters when an actor is required to appear in some state of undress. For example, I think the men and women in Mama Mia and American Psycho are/were probably more conscientious about going to the gym than they might normally be.
One case where I felt an actor made a 'wrong' choice regarding their physical appearance was a show I saw at The Bailiwick in Chicago, during their annual Pride series. The play was about an older man in a relationship with a much younger man, and the play required both actors to be nude in the second act. The actor playing the older man obviously spent some time at the gym to look good without his clothes. Unfortunately, his semi-buff appearance looked and seemed very unrealistic within the context of the characters. It fought against the character's (verbalized) insecurities about being with such a young man, and made the younger character's responses of acceptance seem less 'true' (because the older actor looked pretty darned good, and seemed physically appealing to any age).
Updated On: 7/26/16 at 11:21 AM