tendonitis is an overuse injury.
tennis elbow is elbow tendonitis and comes from overuse of the joint.
if you don't rest it you will screw it up. as for how long, it depends. could take days if you hae caught it early enough but not likely. for a better idea visit a doctor. i won't say because i can't be sure but most cases i know about through my coaching and playing are around 4 - 6 weeks out