How are you doing now? Are you healed?
You're asking a question that may not really have an answer. In a wonderful world, under all the right conditions, gastritis should probably heal within a few weeks. However, in a person with GERD, who is on acid-suppression meds, but who may or may not be following a GERD diet, who may or may not be taking meds for other conditions (and those meds may impact the gastritis, etc), it may be weeks, to months, to years for gastritis to clear up, unfortunately. It's a very individual thing, and something that can be very hard to predict.