Well I bought 5kg for my 150mm Rhino Pro thinking it would be enough, but it only filled it halfway up, meaning I need another 5kg. This shits getting expensive, but I suppose it's cheaper than buying a new CF.
To save money I'm tempted to just top it up with the old activated carbon.
I wouldn't advise re-using the old carbon, especially if you're replacing it because it's 'spent'. If you really have no option though it would be better to thoroughly mix the correct amount of old carbon in with the new carbon and then refill the filter housing using the mixture.
It's lifespan will be reduced and there's also no guarantee it will filter out 100% of the odours.
Is it really worth saving £40 or £50 to risk getting busted 'cos the filter wasn't working properly?