Get a decibel meter or at least a decibel meter app on your phone. Obviously it's not going to be totally accurate since holding it up to your phone is not a good acoustic coupler but at the very least it will give you a ballpark. 85dB and up is already risking long-term hearing damage. Google "noise induced hearing loss" and you'll get some guidelines for how long you can safely listen at any given volume - but also keep in mind that those guidelines are statistical averages that aren't applicable to everybody, and certainly not someone with less than healthy ears. Also, if your headphone has a lot of peaks in the frequency response - as Grados tend to do - the amount of hearing damage they're likely to do will actually be higher than what a totally flat-sounding transducer will do at the same average volume.
Also, if you every hear ringing in your ears, or if your ears ever feel like they're stuffed with cotton after a long listening session, you generally should back off the volume for a bit. Ringing in your ears isn't only tied to volume, there's a large amount of physiological factors involved, but it's generally a good idea to listen to your symptoms and try to eliminate potential causes.