Too little revenue and you raise "hobby loss" eyebrows, and too much revenue with large loss carry forwards and you'll get looked at pretty quick. (I'd say 3-5 years.)
This is 100% opinion and educated guesses, but things that will matter are:
1) Is it the primary source of income for the family (or individual if filing single.)
2) Is it the spouse's Sch C and the taxpayer has a huge W2? (This is lower the flags a little bit, as the situation makes sense.)
3) Is the Sch C an extension of the main occupation? IE: I'm a CPA, that is what is listed on my occupation line. If I was to run a side business doing a couple dozen tax returns and the Sch C is just my side piece I'm less likely to get hassled, but I'm less likely to get hassled because I'm a CPA too.
In short, I really can't give a concrete answer but it depends on a few things. A Real Estate professional who is showing 3 or 4 years of 70-130k losses, and not paying any self employment taxes on the commissions is going to get audited, without question, eventually. Someone taking losses for their hobby of selling wood carvings when their main job is a electrician, may not get audited if the losses are 1.5k for awhile, but end of the day, they are breaking rules.