Depends on what genre (as many people have said), but many "great" songs were written very quickly. If writing is slow or becoming laborious, then I say drop it.
I generally noodle about, over time I have built up a number of riffs or hooks I like, then use them as a starting point, and fill in verses, bridges etc. If you don't have most of the songs structure fleshed out in the first 5 minutes, then leave it, and start with another riff.