Not that long, should be about 1-hour at most, but note that not all devices might connect automatically.
If you have already waiting for more than 1-hours then the general recommendation is then to perform a power cycle of devices. It is usually battery-powered devices that will give you problems which will be resolved by simply by removing and replacing the battery.
If some devices are still giving you issues then then it might be interference issue so try manually re-pairing those devices in the ZHA integration after following this → https://community.home-assistant.io/t/guide-for-zigbee-interference-avoidance-and-network-range-coverage-optimization/515752
Seamless migration only works if you were already using the ZHA integration with your ConBee/RaspBee, as then you are migrating only changing the Zigbee Coordinator and not actually replacing the whole Zigbee gateway application → https://skyconnect.home-assistant.io/migrate-zigbee-zha-only/
There is no seamless migration from another Zigbee gateway application like deCONZ/Phoscon, as it does not migrate and deCONZ/Phoscon database or deCONZ/Phoscon integration the which contains all metadata aboout devices and entities, so going from is not meant to be a seamless migration, as it says so at the top there → https://skyconnect.home-assistant.io/migrate-conbee/
“Note: this procedure only helps migrate basic network settings to your Home Assistant SkyConnect, customizations like device names will be lost and automations will need to be updated. Most powered devices like lightbulbs will be re-discovered over time (you can speed this up by rebooting the device) but your battery-powered devices may need to be re-joined to the network for the migration to complete. There is currently no migration path to migrate all settings and devices.”
That is, you have to understand that this set up a completely new Zigbee network with the old name so is only a partial migration that is meant to make it a little easier since you do not need to run around your house and manually factory reset all your devices individually before manually putting them into pairing-mode one-by-one in order to join each and every device you got.
Hence that step 31 is needed since you migrated to a completely different Zigbee gateway application:
Reconfigure automations and dashboards
All migrated devices have new entity IDs. This means you will need to manually reconfigure automations and dashboard cards.
The names that you manually assigned to your devices won’t have been migrated. You will have to rename the devices again.