There is no rule about how often you should clean your stove. However, the simple fact is that a dirty stove is likely to make a dusty mess in your home, whereas a clean one will not. Furthermore, a well maintained stove will also burn more efficiently and be safer to operate than one which is clogged with ash. It may not be the most enjoyable of jobs, but cleaning your stove on a regular basis will prolong its lifespan, save you money and keep your home safe.
In order to clean your stove properly, you will need to remove the internal parts and clean these separately. Start with the inside, by hoovering and brushing it out to remove ash and dirt build up and then move onto the smaller parts. Whilst you have the parts out of the burner, it is wise to check them for any damage or cracks and, if you find any, contact your local retailer or search online for good-quality wood burning stove spares. For those that are in good condition, a small brush will be all that is needed to remove the ash. No matter how often you clean your burner, it is likely that, at some point, you will need to invest in some wood burner spares, but regular cleaning is the surest way minimise the frequency of this being necessary.
Next, clean the grate with a hard brush, the firebricks with a soft one, and then polish the glass front with some newspaper or paper towels. Finally, consider sprucing up the outside with a lick of paint and your stove will back to looking, as well as functioning, as good as new.