How soon to re-apply Frontline - I can still see fleas!?
As long as you applied the Frontline properly, it will be killing off the fleas. They take about 24 hours to die. While they are dying, they are very easy to spot on the animal. Healthy fleas are more difficult to see, because they hide better. As fleas are dying and dropping off, more "new" fleas are jumping on your cat from the environment. This is a very bad year for fleas. Most of the flea population affecting your cat is not on your cat, but is actually in carpets and nooks and crannies between floor boards etc. They replenish the fleas on your cat all the time.
You should wait 4 weeks between applications on your cat, but you should vacuum all carpets regularly and use "RIP Fleas" or something similar around the house. It still takes quite a few weeks for most people to get rid of all the fleas.