how much do these go for? how frequently do you need to change them? I play about an hour a day and have very good hygiene (aka wash hands before and after playing etc)

There's no rule when to change them, so change them when you want to, or think you should. After a couple of changes, you'll start to just know when you need to change your strings.
Actually, I go by Dave, but there are already too many Daves on this forum.

Fender MIM Stratocaster
Fender Jaguar Bass
Epiphone EJ200 Super Jumbo
Fender Excelsior 13w
Acoustic B300HD (with matching 1x12 cab)
NYC Big Muff Pi
It all depends, some people like the sound of old strings and keep them until they break or rust, others change them as often as they can I knew one person who changed them once a week, I kinda prefer sound of older strings, so its really all up to you