My desktop's monitor is about 100 dpi and it looks fine. I have a laptop with a 125dpi screen and a netbook with a 150dpi screen, and it looks fine there, too.
Just tried it under a Debian chroot on my phone (N900) and it was tiny but very readable on the 250DPI screen as well.
As this is entirely subjective, the answer for some people might be that it's unusable above 75dpi, while for me it's fine up to >3x that.
