You should quit trying only after two conditions have been met: 1.) You've given yourself a legitimate shot. 2.) Trying is no longer fun.
Terry RossioMost aspiring screenwriters simply don't spend enough time choosing their concept. It's by far the most common mistake I see in spec scripts. The writer has lost the race right from the gate. Months - sometimes years - are lost trying to elevate a film idea that by its nature probably had no hope of ever becoming a movie.
Terry Rossio