Assuming you do NOT step back as your print size increases, ie you continue to peer at the picture from about 15cm, and using proper post-processing, i think you'd start to see (resolution) degradation when your print around 17 inches on the long side, and around 40 inches it'll likely be unaccepatable.
If you DO step back appropriately as the print size enlarges, with proper post-pro, it'll blow to almost any size you can afford (in terms of $$).
It also depends on subject matter. Pictures which generally entice people to press their noses up against to see fine detail, like large group portraits, usually enlarge less gracefully, since missing datail cannot be put in. Other subjects, like clouds, can be blown up very big w/o much problem.