My Thoughts & What I Need

[Phoenix]

Since Onyxia is on a long respawn timer per server, what is the possibility that many people, let alone guilds can get their scales?

I don't know exactly what you mean here, but I'm going to assume (most likely wrongly) that you have misunderstood things.

Onyxia's Lair is an instance, you can do this instance whenever you want (assuming that all the raid members have the key). You can try over and over and over again, without having to wait for anything. However, once you kill her, you're "locked" into a raid instance which basically means that everytime you enter the Onyxia instance portal, you enter the one where you've already killed her - which means that once you've killed her, you can't kill her again until you're no longer "locked in". The locking lasts 5 days for Onyxia, I think.