For me, it's when it starts feeling too much like work, or when the hassles outweigh the rewards/entertainment value/other benefits.
For the past year, I followed a blog involving old CRPGs (computer roleplaying games) called The CRPG Addict. He started off strong, and having played many of the games he blogged about (the Might and Magic, SSI Gold Box series, and Wasteland being particular favorites of mine) I found it entertaining and engaging.
Since this August, however, his posts became slower, with more time in between. Life gets in the way, I can understand that, but whenever he did post, he sounded almost put-upon, if not downright snippy. I had chalked it up to life, work etc. but I also couldn't help but notice he wasn't having as much fun with it anymore. Finally, last week, he'd called it quits.
Was I disappointed? Sure I was. I loved reading his playthroughs and reviews of games. some of them were old favorites, some I'd never finished, and some I'd never even heard of. It was a glimpse into a past that I'd never experienced, and a chance to relive some special moments of my own. But in the end, it wasn't fun, nor enjoyable for him, and it was really coming across in his posts. I think he did the right thing.
Last night, Dianne (my partner) and I made the decision to stop playing World of Warcraft, and MMORPGs altogether. The whole thing just became a chore. We're casual players, we play for fun. Raiding is fun, but it wasn't our do-all and be all of the game. Dungeons were fun, but only when the other players weren't being anal orifices. Unfortunately, finding nice players who weren't hidebound into a particular style of play (and hounding you about it when you dared to do something different) was becoming akin to finding an epic drop on a level 10 Kobold. And when we weren't getting hassled, we saw it happening to other players.
Blizzard, alas, doesn't care about this sort of thing. They're too busy banning people over stupid things, instead of banning the level 85 jerk who's griefing noobs in a level 20 area. Which makes no sense to me, because either way, they're losing a player, so why not ban the jerk making other players miserable?
Bottom line, it and other online games stopped being fun. I work in customer service; I can get screamed at for a living. The last thing I want is to be doing something that is ostensibly fun and getting screamed at there, too.