I just lost 50lb with diet alone this year - I just didn't feel like exercising, so allowed myself not to bother
But historically I've generally been pretty active, and in the last two months after I got to my maintenance weight, I've started cranking up the exercise again (and it's easy because I really feel like doing it now). Joined a gym about 3 weeks ago and I started to feel the difference with the exercise quickly - maybe after about 3 weeks I started to feel noticably stronger in terms of leg/arm/shoulder muscles. Fitness is a bit more difficult to put a timescale on because one day I feel great and the next day it can feel really hard, and I haven't really done heaps of repeat sessions that I can gauge that one by, but again i'd estimate a few weeks to get a noticeable improvement.