IMO you can start proofing a behavior as soon as the dog can do it reliably on cue in a way that makes you 100% happy.
If the dog is still hesitating (suggesting that he's not totally crystal clear on what to do) or the behavior is not exactly where you want it (this mostly just matters if you're planning to go into competition, but even if you're not, it's a good habit to learn anyway), then work on that first.
If he's clear and fast and precise, then you can gradually
add distractions and increase the duration. The rule of thumb for any Stay proofing (which includes this) is to be cognizant of three D's: distance, distractions, duration. Raise each one gradually and be willing to drop the other two to tolerable levels as you increase whichever one you're working on at any given time.
Proofing a behavior is an art that requires careful reading of your dog to see what he can and can't handle at any given point in his progression, so take it slow, listen to your dog, and have fun. Eight months is quite a young age, so he's doing great to be at this stage already.