I think I would quit when the niche is way too competitive - and it's simply not worth it to buy the ads to overcome the competition.   OK, one example was when I was trying to make a hosting company - as in a website selling hosting - as opposed to simply being about hosting.  Well, that one just eventually seemed like a loser to me, as my attention drifted to educational and music forums which offer a better shot at things.

Well, on 2nd thought, I think it's possible to win the hosting niche, even for an ordinary guy - but I simply got bored with it - and also hosting brings up the chance of lawsuits, etc. that I feel is not worth the trouble.

Let's say you make a Call of Duty 4 forum and the game is starting to die down. If you still have a couple of active members and the forum is still "kicking", I'm sure that you'll predict that it's going to lose that activity in the coming months. You can, of course, try to sell it and quit. 

You should never start a forum with the mindset that you're going to quit. Best just not to start it at all. 

I'd say that you should quit if:

1. Nobody joins/You do get a few users but they aren't very active.
2. You don't earn any money/You earn a few dollars & cents/pounds & pence but it's not enough.
3. Nobody likes the forum.
4. It's hard to keep users interested.
5. What you're using/doing is outdated.

That's a list of reasons that I think is the best time to stop doing your web-project. There are of course, plenty of other reasons to quit. These are just based on what I think.

i am in the process of quitting my project. I and one of my friend is posting regularly and i don't see any new registrations or activity. I will run for few more months then i will make a decision.

As sad at it sounds, I'd suggest it is time to quit webmastering a website when any of the following occur:

1. You have inactive users and nobody hardly registers or posts
2. No profits or donations have been earned to help contribute towards Web hosting and maintenance costs
3. No one really likes the forum, website or blog anymore.

Hope this helps you. 

I would quit a project at that time when I realize that the forum is not going to happen. I mean when it's obvious that nobody joins the forum and there is too little to none activity going around there. Of course that to establish a good community you will need time but if you see after couple months that the activity is almost none I think it's the right time to leave.

I'd like to point out how easy it is just to give up. Failure only teaches us and learning by it is essential.

You'll succeed if you learn by your mistakes and listen to others around you, online or in real life. 

When you no longer have the energy or will to keep doing it and someone makes an offer to buy your site. I've seen a lot of people quit due to that.

I seem to have great success with adult forums, but not so much with "regular" ones.   I don't quit too easily tho. hehe

This week i closed 1 project as i felt i don't have the potential or motivation to do it. So closed the hosting, domain.

I only ever try to start projects in areas that I have a personal interest in. Like if I was to start another one now I'd pop onto a gaming forum and get that going because that's what I'm into. At least in some ways, that stops me having as much of a risk as losing interest a bit down the line. I always set myself goals, so in six months I want to hit X amount of members, X amount of posts and when I sit down six months later, I measure my progress against that. If it's way off where I want to be, I'll likely shut it down so I don't flog a dead horse.