How long would revolution or frontline or other flea products last for a dog?

Just wondering how long these products last to give the dog flea protection? I am really concidering it now since one of the dogs got fleas i think. and what cost is it in canadian dollars, I think its around 80.00, but not sure? Does it last a year?

I normally use frontline but after speaking with my vet and using it for a long time they seem to become resistant to it and you may have to use it every 2 weeks. You are better off going with revolution or advocate if you have it. I am in Australia and it costs about 40 aus dollars for 3 frontline.