Theoretically infinite!
Usually, the second harmonic is stronger than the first, so you don't really need anything below 160 Hz for guitar, but some like more fundamental. Then, from the second harmonic on, the level decreases, eventually becoming inaudible. Most guitar speakers don't have any significant response above 5K, and that's a good thing if you drive your amp into distortion. Those last two octaves sound really raunchy, and not in a good way. On the other hand, if you're running clean, the those last two octaves can give you sparkle you'll never get otherwise. Start overdriving, though, and it gets nasty quick! Essentially, the 5K rolloff acts as a filter, or tone control, that makes distorted amps sound good. Rolling off at a lower frequency will make the tone darker, which some people like.