How about very little and focusing instead on passing the course?
You will be too busy, esp full-time to commit to actual flying. However before commencing ATPL theory there is a wealth of stuff you can do. Fly more complex stuff than a basic 152/PA28; tailwheel, wobbly prop, floppy undercart, farm stripping ... All helps build your experience, captaincy and understanding of aircraft systems.
I worry when I see recently passed PPLs go straight into ATPL theory without actually having used their "licence to learn"