I use a Hardy Marquis 3 for most of my salmon fishing and, even when it's not filled to capacity, I get at least 250yds of 30lb backing. Some of the spools have well over 300yds.
I once walked all the backing out in a field when I was changing from RHW to LHW. When you see well over 300yds of backing unwound in a straight line you realise that if a large fish takes that amount of backing it's probably three pools down and off the hook.
However, as Willie Gunn says, you can never have too much backing. I like at least 200yds, which usually covers you if you cannot follow a fish up/downstream because of bankside obstructions. Knowing I have this much backing let's me fish in comfort and with confidence, and that is the key.