Headlice eggs take 7-10 days to hatch.
I don't think there is any treatment that kills eggs before they hatch on a 100% basis.
And headlice can get old enough to start breeding in 7 days (according to NHS website).
After getting driven to distraction trying to clear DD's heads I put all that together - and so the only way I've found to kill an infestation is to coat hair in a very effective live lice killer (Hedrin has worked for me but there are alternatives I'm sure) 3 times: 6 days in between the times.
This means you don't get any headlice getting old enough to breed, and you've ensured any eggs around at the start of the treatments have hatched.
Oh and lice can survive for 24 hours off people's heads so you need to wash bedding and clothes every time you treat.
If you don't treat on that kind of pattern, it's very easy to end up getting reinfested because you've missed enough eggs to get breeding going again.
And you still need to comb all the hatched egg cases out.
And you still need to keep on checking weekly if you reckon reinfestation is a potential problem
It is all so veeeeeeeery tedious.
Personally I'm amazed nits aren't even more common...