To learn more about what's happening on the International Space Station at any given time, check out NASA's frequently-updated ISS blog or their page about the research and development work happening on ISS.
Depends on your body type and metabolism and your routine. But usually it takes around 3 to 4 weeks for visible results. For me it took me a couple of months to actually see any progress visually.
When you exercise your leg muscles, they will grow a little, but you will mostly see the effect immediately after exercise as the muscles swell to bring in nutrients and expel waste. This effect goes away after 30 minutes to an hour. As the muscles recover, they build new fiber and also use it more efficiently. The result is unlikely to be visible as a larger muscle, but rather a leaner, toned muscle. If you exercise enough and watch your diet, you will lose fat as you gain muscle.