Pepcid works fairly quickly. So definitely start taking it ASAP. It doesn’t take a month to start working.
Not trying to frighten you but without getting that acid/acid reflux under control you’re risking damage to the esophageal sphincter, Barrett’s Esophagus, stomach ulcers and tooth erosion.
Omeprazole works better and longer. (Prilosec, Nexium) Each pill generally will last 24 hours. Pepcid works faster than Omeprazole but for not as long. So if you find the Pepcid isn’t working as well you can switch. But again, check with your transplant team. You have frequent blood work done anyway, so checking on reactions to any of these products will be noticed quickly. Don’t be afraid to try them!

Gaviscon is a great over the counter acid reducer because it coats the esophagus and stomach lining. And you can use this as the same time as the long term products.

Avoid eating acid producing foods. https://www.healthline.com/nutrition/acidic-foods

Don’t eat several hours before bed and don’t drink a lot of liquid before bed either. You don’t want your stomach full.
Try sleeping on your left side. https://www.everydayhealth.com/gerd/managing/sleep-tips-for-people-with-gerd.aspx.

Avoid caffeinated or carbonated beverages.
Elevate the head of your bed or prop with pillows so you’re not lying flat.
These are a few helpful hints to avoid acid reflux.

Do you take any over the counter products in the meantime?

I am not sure how long it takes normally. However, for me I was better in about a week. Some people take longer to respond.