This is a good point! - How long is too long...

For a Blue Tarp Roof? http://www.thestar.com/yourtoronto/the_fixer/2013/06/19/plasticwrapped_roof_has_passed_its_bestbefore_date_the_fixer.html

Of course if a family is beset by misfortune what are you going to do they shouldn't be punished for doing the best they can but isn't there a law for the rest of them?

I've seen houses where the tarps are totally dissolved.

Nothing left but traces of blue on the old shingles.

Maybe around 6 months should be the limit under normal circumstances with special exceptions for storm ravaged areas.