The frequency will depend on a number of factors including how often the stove is used, the moisture content of the wood used and the type of chimney. A chimney sweep should be able to advise you when they have swept the chimney.
At the very least the chimney should be swept at the start of the heating season. The Guild of Master Sweeps recommends that the chimney be swept quarterly during the period when the stove is lit.
It is recommended that you engage a trained chimney sweep to clean your chimney.
A chimney is designed to safely transmit the products of combustion to the atmosphere. If you are at all concerned about the integrity of your chimney you should seek professional advice right away and have it repaired by a competent person.
There are three associations representing chimney sweeps and engineers:
Each association maintains a register of members so it should be relatively easy to find a chimney sweep or engineer in your area.