Seth Godin wrote a great book on this exact question. How to know when you should push through the dip and when you should decide to quit. It's an insightful read:<p><a href="http://www.amazon.com/Dip-Little-Book-Teaches-Stick/dp/1591841666/ref=sr_1_1?ie=UTF8&qid=1323703786&sr=8-1" rel="nofollow">http://www.amazon.com/Dip-Little-Book-Teaches-Stick/dp/15918...</a>
I still wonder if AirBnB would have made it through the trough of sorrow following this model.<p>Regardless, a great framework to consider. Startups can drag on indefinitely and we should consider the opportunity cost of hanging on to a single project. It's okay to move on, and it's okay to fail.
I think a big thing is to be able to see growth. If you have 100 customers last month, and increased to 150 this month, even if you are only charging $10/mo, at least you are seeing that it'll take you X more months to hit decent revenue<p>If on the other hand, your whole future planning is based on some huge event that'll make you, then you need to think long and hard on whether or not it's likely to happen. If it's been 6 months, and you've done everything in your power to make that happen with no results...then it's probably best to throw in the towel, since it's unlikely that it'll actually happen
If what you built is a consumer product, and accessible to friends and family - then you know you need to move on when friends after being coaxed into using your product still don't give a damn.