Pretty often... I've had mine for about three days, because it's one of my favourites. I usually change them everyday, multiple times a day, o every segundo día though. :3
Hmmmm pretty often till i find one i amor then i'll leave it that for about a week...2 weeks maybe like my current one i amor it soooo imma keep it for a while!!(btw its btt genderbent:)
I just change mine when I find a better one. Like if tu look at my gallery I have multiple iconos of the same person then another person and so on. When ever I get into a new anime o Fangirl about a character I switch my icono to them. Earlier today it was Izaya Orihara, then Kida Masaomi, now Akito Tagaki~