It’s usually more like 20 minutes, although it can go faster, and they aren’t actually regenerating the mesh at that time. Instead, the individual zigbee end Devices are going into “panic mode“ because they can’t find the hub. Then after the hub comes back online, that’s when the mesh gets regenerated as each individual device tries to find the best parent. That process can take anywhere from a few minutes to a few hours. You can feel confident that it will be done by the next day, but how much sooner is hard to predict.
There are many resources on how zigbee 3.0 works, so you can just Google that. Just look for reputable sources like the zigbee alliance itself, IEEE, Digi, etc. There’s a lot of misunderstanding out there.
As far as how it all works within a smartthings context, try the following FAQ. It should help. 
A Guide to Wireless Range & Repeaters
As it happens Hubitat has a good zigbee discussion which nicely hits the middle mark between mass consumer and network geek. After you read the FAQ linked to above, it might be a good second step. Everything they’re saying there would apply to smartthings as well except that where they have a limit of 32 directly connected nonrepeating Devices, smartthings has a limit of 64 for the V2 and V3 hubs.
https://docs.hubitat.com/index.php?title=How_to_Build_a_Solid_Zigbee_Mesh
Also, if you’re looking for a good two sheeter introduction to zigbee, something that looks respectably technical but is easy to understand, The following is old but still applies. It’s from Digi, but it’s so old that now you have to find it on other sites. Anyway, it does a good overview of zigbee mesh. It makes it sound a little smarter than it actually is in practice, particularly in a smartthings implementation, but it’s theoretically sound.
In zigbee, a “coordinator” is the same as a hub: there’s only one, and it establishes the network and owns the security keys. A “router” is a repeater. So it could be a light switch, a plug-in sensor: it doesn’t have any special network functions other than being able to pass along messages from other devices. and an “end device“ is anything which isn’t a coordinator and isn’t necessarily a router, like a battery powered sensor. But zigbee Routers are also typically End Devices. 