water conditioner and charcoal filter

how long is a charcoal filter for a family of nine suppose to last for and what size should it be. It was just changed by Rainsoft and was one year from last changeout. It was sludge and a foul odor was coming from it. If the was flow is much greater with nine in the home vice only three when initially installed then should the filter be larger and should the conditioner be replaced too. It is a Rainsoft system from many years ago.