I don't use charcoal but what I have read is that it is only effective for a very short time. I have read from one week to one month. I suppose that partly depends on how much charcoal filtering how much stuff out of how much water. (I really don't know about amounts. you would assume that the manufacturers provide the amounts they do for some good reason but, ya never know)
Charcoal is used to remove 'chemicals' (such as medications for instance) and somehow, odors from a tank.
A filter system will also generally provide mechanical filtration ( taking floaties out of thewater so that it looks cleaner)
And biological filtration, which is what keeps the ammonia and nitrite levels in check.
SO, I don't know what type of filter you have or what else is in it besides carbon in bag. If that is all there is to the filter, then It could be that you would loose a good bit of biofiltration if you dump the charcoal.
( even when it isnt removing chemicals anymore, it would be providing a home for the biofilterring bacteria)
I assume this is why you reused some of the old charcoal?
you could tell that by testing the ammonia and nitrate levels in your tank before and after you "change" the filter I guess.
If you dont get a better answer than this, id suggest you try the question as a new thread *S