I have 15 bags that I can think of at this moment. 4 are medium sized purses, 2 are big work totes, 1 is a tiny work bag that I take just to lunches and meetings, 5 are wristlets/small evening bags and 3 are casual totes that I only use for the beach or pool.
My most frequently used bags are my 3 work bags, since that's all I need for 5 days of the typical week. One of my big work totes is black and one is camel, so I switch daily as necessary depending on what outfit I'm wearing. I don't always have to switch from the previous day's bag, but if I have to I do. I probably change my work bag 2-3 times a week.
On the weekends during the day, I use one of my medium sized purses. I almost always use the same one, but of course that depends on my outfit as well.
In the evening, I usually use my favorite gold wristlet unless it's a super formal occasion, in which case I use one of my fancier evening bags.
So on the whole, I would say I switch bags at least 5-6 times a week, and only about a third of my collection is really in rotation at any given time -- the other two-thirds are bags I keep around for specific events like beach vacations, formal dinners or just when I get bored, but they probably only get used a few times each per year.
~Ash