Depends on what it is.
For Reta I don't have any issues with 98%+, haven't tried under 99% for Sema and Tirz so I can't say for sure - But probably the same and maybe even lower, though I try to draw my personal line at 98%.
But, let's be honest, I'd probably inject something with 80% purity if I had it lying around and was confident in it not being dangerous - why let it go to waste.
For HgH on the other hand I need pharma grade, or my wrists betray me at 6IU+ per day and the pain from stiff joints is so bad I can barely go to the gym after a 45minute wrist warmup.
For all other peptides, I haven't really played around with them enough to make a comprehensive list of which makes me feel low purity. But 98% or 99% is a good rule to live by as a customer I believe.