Basically, time played gives far much more exp than anything else in the new progression system, so in hero mastery you actually don’t lose out on that much exp compared to quickplay even though time played is the only stat you can level up in it.
I tested and there does not seem to be any form of time limit or kicked for afk in the mode (I only tested it for 60 seconds though because I don’t actually want to mess up my real stats and level). There also doesn’t appear to be any exp cap for a single session either since while playing hero mastery legit I tried the one rein course for an hour straight and got 1600 time played from it and the level up.
But if you can actually just complete a course, stand next to the finish line, go afk for a few hours , the cross the finish, that’d kind of break the whole hero level system. OW1 used to have exp in custom games before people started taking advantage of it.
Of course it isn’t nearly as important here. An hour or s oof exp in OW1 gave you a lootbox worth $1 and upgrades your portrait that’s very visible both on your HUD and on the leaderboard. Meanwhile an hour of exp in hero progression gives you 1/4 of the exp (I’m not joking that’s how bad it gets at level 20) needed to complete 1/ 8 of a weekly challenge that gives you $0.10 if you complete 11 other weekly challenges, and occasionally you get a hero title or player icon or some other garbage
tl;dr - it’s not very useful, but it’s kind of interesting that it exists