As a kid I used to have a lot of free time, so the main reason I stopped playing a game was not it's difficulty but if I liked it or not. For example, Bart vs the Space Mutants or Top Gun 2, I thought those games were hard and I didn't like them at all so I never wasted too much time with them.... TMNT, Ninja Gaiden and Battletoads I LOVE all of them and played it for a thousand hrs. I'm very proud I could finish them except Battletoads, but even if the game is very difficult I really enjoy playing it =)
Nowadays I'm very busy so a very hard game can become more frustrating but I still downloaded some Nes games I never played before like Castlevania III, and after so much swearing and frustration I could finally beat it.... I can't imagine playing it without Wii save point though.
Generally I stop playing a game not by it's difficulty but if they are boring.
Castlevania Judgment FC: 1161 - 3389 - 1512
3DS Friend Code: 3480-2746-6289
Wii Friend Code: 4268-9719-1932-3069