it's three times a week, so you take a day or two rest between each 200-250 serve session.
If you are not fit, start with 100 serves sessions. Surely if you are playing tennis regularly you should be able to make 100-150 serves in an hour and then call it a day? Take breaks every 50 serves if you need to.
Disclaimer: I'm NOT a coach...
Real tennis: https://www.youtube.com/watch?v=yDqnkLJ9BtM