The flea lifecycle can take 3 - 4 weeks to complete depending on environmental conditions. (hartz.com)
Not all shampoos will be able to kill both adult fleas and the eggs, so in this case, you may need to use a spot - on treatment as well to ensure that the flea lifecycle is correctly halted. (totallygoldens.com)
Spot - on treatment also has its limitations, as it usually either kills the adult fleas (thus leaving the eggs and larvae to hatch and restart the flea lifecycle), or just the eggs — again, you may need also to use shampoo to kill the adult fleas in this case. (totallygoldens.com)