I have the same fridge and on average we are defrosting it every couple of weeks. It always happens when the fridge is full (i.e. after a sainsburys delivery).
We are getting a new kitchen in March so hoping to keep it going until then. We might invest in a backup for the garage.
Something doesn't sound right there, particularly if it's frost free. Bought our Bosch fridge/freezer nearly 8 years ago and haven't had to defrost it once.