Probably not long. The timer is set in your ZD. AP will reboot if it cannot contact the ZD, by default in 30 min. You may extend that time, but in most cases, if there is authentication on your WLANs, the wifi will fail almost immediately, since authentication will fail.
I would hang the new APs. They will not be able to connect to the ZD, because they will not be able to downgrade to the ZD version. Old APs would continue to work.
Upgrade the ZD. Now the new APs will start connecting and taking config, while the old ones will fail.
That's all depends if the license on your ZD supports the two types of APs and their numbers 😞