What is the recommend volume of a guitar amp when recording through a mic? I am using a Shure SM57 or a MXL 990 mic.
As loud as you're comfortable with, but the louder, the better. It's best for sound quality to saturate your recording medium with sound.
Hi, I'm Peter
As loud as possible (while keeping tone quality). The lower the mic volume the less noise that will be picked up, making your tone crisper. When your amp is that loud it also takes in some room acoustics, which give a warm sounding natural tone...

For hard strummers or blinding fast pickers, physical pick noise is cut down.
Thanks. I was asking because, when I record I usually have the volume low and, my recordings sound awkward.

Another question, how close should the mic be? I usually put it like 6 inches away from the amp grill. Is that too far?
Mess around with mike placement. It's really an art. Try it dead center touching your speaker cloth. Try it off-center. Try it a distance away. You can change the tonal properties of the recording (sometimes substantially) by mic positioning. BTW, if you're in a room that has natural reverberation, the further you place the mic from the sound source, the more the natural reverb will show up in the recording. If you want to really have fun with that, try recording in a bathroom.
Hi, I'm Peter