I had been hitting buckets of balls for 1 - 1.5 hours at a time, often swinging with full strength over a period of 2-3 weeks, every other day. Shoulder felt a bit sore, but I ignored it. After a 10 min. session a week ago, the shoulder becamse so painful I stopped. It's in a sling now and I can't type or use a mouse for more than 10 min. without taking a break. This is affecting my work. So, if you're going to work on your serve: 1. Limit to 30 minutes at a time. 2. Ice your shoulder afterward. 3. If you feel any pain in your shoulder, just stop. We've all played through tennis elbow, sore feet, sore muscles, etc., but shoulders are pretty serious business, as I'm learning now.