I change bags once a week, unless I'm going somewhere special. I have one of those handbag liner "thingy" (it looks like a purse and it holds all your main essentials, wallet, mirror, tissue etc.) where all you have to do is remove it and place it in the handbag of your choice. It's great for people who like to change purses often, not to mention the time it saves you.
I suck at changing up my bags. I usually only change for occasion. On evenings I'll take out a smaller evening bag, but during the day, I always go back to a Chanel tote bag. It's my standby, the most comfortable and convenient to carry.