I’ve fired up several engines below freezing and one was in the low teens. It had to get done. They all still run and don’t leak. You don’t have to wait.
That's been my experience too. I can't think of a time that not waiting caused me a leak. Every leak leaked because I did a poor prep job, or there was a bigger issue (blow by).
The time to assembly has always been enough time for RTV to set for me.
IIRC, it's endothermic. Gets colder as it cures, but only slightly. Last engine I built, it was 10 degrees and getting colder each day. To be fair, it took me 2 days to get the engine in the car and fire it up for break-in. But when I redid the intake, it was maybe 20F out when I installed it and the thermostat and I fired it up right after I got the belts on it. No leaks, no problem.