I guess we must have it pretty easy, because we never plan who is going to do what - but our place stays pretty neat most of the time and the workload seems to even itself out. We work different schedules most of the time so it's nice to come home to a clean house when my SO has been home all day! Lucky I guess
If you can keep yourself lighthearted when you bring this type of stuff up it would probably help. Whether it's the first time you asked or the most recent, people in general have a tendancy to react in a defensive manner when confronted offensively. For example, she's at work and you clean for the afternoon and when she gets home it's nice and tidy - you say "Pretty Good don't you think? Next time it's your turn!" in a lighthearted tone.
Also, you have been going out for a long time, it might be time to start thinking more "ours" and less "mine" like maleficient said. Your GF might take that kind of stuff personally.