I change mine about every three months.
I use an electric one with those blue bristles so I change the head when the bristles have turned white - probably around 3 months.
Quite a lot because I saw on a TV programme once that when you poo and flush the loo poo particles fly up in the air and settle on your toothbrush and anything else close by! GROSS
I always buy toothbrushes when they are on offer 2 for 1 etc so I have a good stock.
same here, i buy lots of replacement heads when they are on offer and replace when bristles go white........ or when someone knocks it in the sink without admiting it and when i unsuspectingly use it it tastes of soap as happened this very morning !!!!!