It depends a lot on your system. How big is the tank? How much LR do you have in it? Do you have LR in the sump or refugium? How long has the system been running? What kind of bio-load do you have in the tank (how many fish)?
I could pull 50% or more of the LR out of my tank and I wouldn't worry at all, but I have a lot of LR and extra in the sump. If you have a small amount of LR in a big tank, then thegrun is probably right. But I'd say that's an extremely conservative take on your question. If you have a good amount of LR and a light bio-load or the system has been running for a year or more, I'd be a lot less concerned and would guess 25% or more could be swapped out.
After you do the change, test ammonia, nitrite and nitrates often (every day for 3 or 4 days) and see if you get any kind of a spike. You may get a small new cycle. If you get a big spike in ammonia, you can reduce the effect with a water change, or by adding other chemicals. But I hate adding ANY chemicals into my tanks.
