Booster would probably waste it. He's not very good at the whole time travely thing yet.
If he had Rip Hunter giving him advice (not help, just advice), and access to the Time Sphere, well, he'd destroy Kang with that week of prep. Or if this fight had the Booster he was going to become, rather than the type of guy he currently is now...
"Prep" is kind of weird concept for two guys who have access to time travel technology. Kang doesn't get any according to this fight, but that doesn't mean he couldn't just jump out into the time stream the instant he figures out someone's coming for him and use prep anyways.