It depends on subject matter: often portraits can be enlarged much more than say a landscape with lots of fine details.
It depends on technical quality: a shot with camera shake or underexposed will look worse the more it's enlarged.
It depends on the viewers' eyesight, viewing distance, and opinion.
Assuming the technical quality is good, I would think you'd be able to go up to 36"x16" (your resolution implies an odd aspect ratio) by incremental upsizing using bicubic smoother in Photoshop.