I don't really worry about tendonitus. Sometime i'll go up to the second floor of my house and play for like 2 or 3 hours straight on, and when i finish and my hand and forearm have a tired or tense feeling, i see it as a good thing, as it's working those muscles and i'm getting faster at stuff. I thought that tendenitus came from bad habits in picking, like using the elbow while playing instead of just the wrist.