Is it safe to treat lice every few days for weeks on end?

Lice. You need to shampoo with the correct compound like Nix (permethrin) or Rid and then go through each strand of hair with a fine tooth comb so as to remove all the eggs which are tiny and white in appearance. Until you do this correctly it will continue to occur.
Lice. Depends on what you are using; follow the directions carefully. But, if you still have lice, they are probably resistant to what you are using and may need a prescription medication.