This is a question that many guitarists ask, and there is no one definitive answer. It depends on a number of factors, such as the type of amplifier you have, the size of the room you're playing in, and your personal preference.
If you have a small amplifier, it's best to keep the volume at a moderate level so it doesn't get overwhelmed by the sound of the room. If you have a larger amplifier, you can crank up the volume without worrying about overwhelming the room.
As for personal preference, it really depends on what kind of sound you're going for. Some guitarists prefer to keep their amps at a lower volume so they can control their sound more easily, while others like to crank it up and let rip! Ultimately, it's up to you to experiment and find out what level sounds best for your amp and playing style.