As Summer said, it depends on how much I've eaten - usually every 3 - 4 hours and yes, it's only whenever I pee anyway.
As time goes on, you will probably get lazier and less aware of your bag and so, leave it for longer before emptying. Right now you're very conscious of it and so are aware of it filling up - in time that 1/3 full will become 2/3's full before you empty :)