The boot time is variable depending on the platform, most MS switches I'd say are 3-5 minutes, IOS-XE could be longer ~10-15, but all of that can also depend on firmware CS IOS-XE will be slower than regular IOS-XE, etc. To be clear the switches don't gain their config, they will have it already. Unless you are making changes while they are offline they will boot up and not need to apply any new config. If they all boot simultaneously they will come online nearly simultaneously. If you have concerns that devices won't recover correctly if the network is not fully online, I suggest you have them powered off or just inform the end users that they may need to reboot first thing.
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