Hi!
That depends on how you got your master discovery set up. If all the APs discover the master controller via DNS/DHCP options first you´ll probably get a bottle neck in how many APs can be upgraded at a time depending on bandwidth, controller model, if you have RAPs etc, it might delay the process.
You could check out "AP Image Preload" if you have controllers that supports it. That way you can pre-populate your APs with the new firmware image to minimize the maintenance window. Look it up in the User Guide.
Cheers,