To pass a level? Maybe 3-5 minutes.
To replay a level to get gold - if I can spot something I could've done differently or quicker, then I don't mind replaying it. But if I did everything I could think of, and still didn't get gold, I'd give up.
Here, when I play browser games and I can't even pass level 1, I just tend to move to the next game.
That's why I use casual and expert features in my games. With casual, you should pass, with expert, you might not. Players can switch freely, so if they can't pass a level on expert, they can switch to casual, play the level, and then switch back to expert if they want to.