Having the Glans "somewhat" engorged with blood is very helpful to the fastening in process. Do not interpret this as fastening while erect, semi erect I'd say ok. You want to be "mostly" flaccid.
As your wearing your extender, keep an eye on Glans color, temperature. Helpful to loosen strap/noose a bit to allow improved circulation, if you see signs of discoloration, cool Glans temperature. You can always take it off for a quick break & put it back on again.
It is not how many hours you can wear it without taking it off, it is more the total hours worn in a period of time.
If you are not wearing your extender comfortably, then something needs to be changed.