They should certainly be kept for some considerable time. It is the signed hard copy that is the only lawful and authentic record. If you have older paper records and nowhere to store them, principal authorities can often provide archive facilities, usually at no cost.
It's usually not a problem to let the computer record accumulate, as storage space gets ever cheaper.
The relevant statute is the good old Local Government Act 1972, Schedule 12, para 41(1). But it doesn't give a direct answer to the question of how long minutes should be kept.