| Anecdotally, I usually find that when I'm starting to warm up, it takes a few strokes of the bow before I get the usual 'grab'. That might be because of my technique or because of the rosin needing to warm, or something else. However, I do find that it's best to play for a minute or two before deciding how much rosin to add.
In an orchestra situation, I might be more inclined to simply add it, but for solo and practice, I try to be more careful about not using too much rosin or using harder rosin. I have a cake of hard and medium rosin, and I use hard for solo and medium for orchestra. | 
