everyone is different.
the last r/m i was involved with was Double Eagles/Fuse Science. When the merger was finalized, it was still a couple months (i can't remember exactly but i know it was a few months) before the symbol changed from DEGH to DROP.
on the other hand, a stock I was watching and regretfully wasn't a participant, changed within a week. that was SHKZ and now RENU.
like i say, everyone is different.