Question:
What is the approximate time between when a first pop returns? I realize it depends on the can.
But say I fire one shot then a second shot, then wait 10 min, is the first pop going to return?
10 minutes, almost certainly. As mentioned, there are a number of variables, but as soon as there's enough oxygen back in the blast chamber, you get pop.
FRP isn't really an issue with high velocity rifle rounds. It's there, but masked by the higher overall SPL and sonic crack. I have never been able to discern a first shot being louder than subsequent with supersonic rifle rounds beyond standard shot-to-shot variation.
With full sized pistol suppressors, it's noticeable, especially with monocores. Good baffled designs tend to not have nearly as much, if any.
Pistol "K" cans (shorties) are serious offenders without ablatives, first round and subsequent. I have a 5.5" version of my Phoenix XLV .45 cal; it's not hearing safe dry, but with ablative, it's nearly as quiet as the full 9" modular version is without, and the first round is barely louder than subsequent.
FRP can be very noticeable on .22 LR with pistol hosts, as much as 10 dB. And shorty rimfire cans, like pistol K cans, benefit substantially from ablative for first and subsequent shots.