Cash credit has no time limit.
It only disappears when it's used to pay for membership, add-ons or other services that are charged to your cash credit balance - e.g. sending MMS messages, international calls/texts, calls to special numbers such as 1850/1890 etc.
To use any services you do need to buy a membership though.
See here for an explanation of the differences between cash credit and membership:
http://community.48months.ie/t5/Most-Popular-Questions/Everything-about-membership/m-p/24Hope this helps.