Mild glaucoma. Changed from travatan to cosopt due to side effects. After 3 weeks pressure is 13. Next pressure check in 9 weeks. Is that soon enough?

Yes. Your eye doc verified the efficacy of cosopt™ after 3 weeks. As long as your doc is satisfied with your iop, your next pressure check can be as long as 3 to 4 months later.
Yes . That is an appropriate time interval to retest the pressure.