How long should I be using root booster? I'm ...

I never use root boosters. The plant naturally produces all the hormones it needs to grow good roots on its own

Depends whats in it. If it contains just P and K then all though flower is good, Keep an eye on the EC. If it has other stuff like Humic and fulvic acids once every two weeks is perfect and will be a great addition. If it also has mycorrhizae. its a bit wasted every week, really only need this at the start and on transplants as the roots will grow with them on it. Maybe a boost at flower. Also comes down to cost. Some products are quite expensive. I use one that's just bacteria and use it once every two weeks at 2ml/L I also dust the root ball with a dry mycorrhizae at transplant and put some in the hole at seedling stage. Good Luck!

Monkey recommend using there's whole way through so it's probably more common