Cranky at my visor!!!!

Sooo. Left work late last night and got hit with some lovely brisbane rain. Was a nice zenfull ride untill my visor decided after 6 months It no longer felt like being an 'anti fog' variation... Maybe 6 months is its used by date...
How long have people found it takes for you 'Anti fog' to become 'not so anti fog'??
How long have people found it takes for you 'Anti fog' to become 'not so anti fog'??