Head Lice survive less than 1-2 days if they fall off a person and cannot feed; nits cannot hatch and usually die within a week if they are not kept at the same temperature as that found close to the scalp (according to the CDC).
With schools starting, now is the time to educate kids about not sharing clothes, blankets, pillows, hair stuff, combs, etc. It is also a great time to get in a habit of checking for lice and nits.
Some forms of lice have now mutated into Super Lice. Basically this means that they have become resistant to the over the counter treatments. If the OTC treatment does not kill the lice, then this would be a time when you call your doctor or pharmacist and they can make sure the treatment was used correctly and then they might recommend something different if they think the lice are resistant to the first treatment.