Wtf? How much does your girlfriend cost you per month?
If your girlfriend is costing you money, she's not a girlfriend, she's a hooker.
No judgement on that, just be clear where you stand. That means she shouldn't be too put out when you decide you fancy a change next week, but are happy to resume the week after.
Of course, if you're talking stay at home wife with kids, then that's different. But then it's not really the wife that costs the money, it's the kids. You're effectively hiring your wife to do childcare.