None. It is not necessary to fail to succeed.

If you think, the idea is good, give it a shot. See what's the response. If it works, continue else quit.

Key thing is to know when to quit.

That is the key thing. I think perseverance is important, but perseverance through iteration. How long would you give a new web startup to reach 1,000 users? 10,000 users? before you considered it a failure or necessary to quit.