During a Trimming Session you'll have 10 seconds to earn up to 10 rewards for your Main Game. These rewards can be Cards, Characters, Buds and Gems.
10 rewards is the maximum amount of rewards you can earn, so if you hit that number before the time runs out then the Trimming Session will end (as seen below).
If you're not reaching 10 rewards before the timer ends, be sure to upgrade Characters that give you Trimming Power. This will allow you to fill up that green bar along the top faster!