You should see the effects of the increased dose immediately. When Hailey was about 10.5 months we increased her dose to 22.5 mgs when she was going through a string of nightwakings. It seemed to help immediately. We were doing 7.5 mg in the AM and 15 mgs at night.
Personally, I would try and go without the Pepcid for a few days with this increased Prevacid dose. If you notice a huge change without the Pepcid, you can always go back to giving it. The effects from the medicine are immediate and there is no 2 week waiting period if you re-start. I hope this helps.