We recommend descaling every 6 months to a year or when there is a scale build-up.  

A couple of recommended cleaning solutions are Decon DeSCAL and Duraclean Autoclave Solution.

Decon DeSCAL

Duraclean Autoclave Solution

The DeSCAL cleaning solution is a bit stronger than the Duraclean product and I would recommend using it for the hard water stains and tough rust if you have these in your chamber.