Once every few months, we bought them last year and replaced the cake once then when the heating goes off and windows are open 24/7 they go back in the loft. Have just got them out again.
open. your. windows.
(not fully, just now and again, a small amount at night and when drying clothes etc)
the end.
Just installed a nuaire drimaster in the loft, so far so good, we live in an old cottage with damp/ condensation issues. This morning when I woke up there were no drops on any of the windows and the air feels fresher throughout the whole house.
If you have one of these I suggest you have air tiles at the high points in your loft to let the hot air out.