Personally, I usually stop when I win my first bet, or when my stake reaches my loss limit, or when I'm bored with the game.

In my opinion, the right time to stop depends on each gambler. For example, if they're addicted, they'll likely stop when they're all gone. If they win, they'll usually continue until they're satisfied and then lose again. So, in my opinion, the skill of stopping at the right time is likely only achieved by responsible gamblers who aren't overly obsessed with winning.