The rule of thumb depends on who you listen to. PADI, for example says 18 hours is fine after more than a single dive. Guess what. PADI has no bloody idea what sort of diving you just did. You may have just completed an altitude dive, then dives at sea level, or the reverse ,all within 24 hours. You may have just completed a dive where there was a decompression obligation. That 18 hours now is just BS because it pertains to an average PADI, essentially OW dive . It doesn't cover anything more exotic and oddly enough PADI is getting into the realm of Tech. The more time before airborne, the better now. Personally, PADI's rules just don't cut it. Keep your no fly time as long as you can past PADI's recommendations. If you own a altitude/ no fly dive computer, use it's algorithm to calculate your no fly and add some prudent safety. It's just plain smart.
To put it in perspective. If I were to tell you that driving your particular car on a certain road was "absolutely safe and without any risk at all" doing 75 mph but driving it at 76mph was "ok, I suppose", which would you choose?
PADI might be "where the world learns to dive" but the learners need to remember that they are just starting and to continue it just gets more conservative.
One thing I'd like to add here about no fly. I'm not in any way knocking PADI as an organization. I'm stating a simple truth. Their no fly just doesn't hack it and can be potentially dangerous if some conditions are met. Then again, like I said right off the bat, "PADI for example". People need to read the fine print.
Myself, owner of a commercial dive business.