It depends a bit on her age and whether or not she is the first child.
When I had my first child I'd buy something new probably at least every month. But he was my first so we were building up a "library" of age appropriate toys - they are constantly changing and developing in the first year and a bit so there is plenty to buy for a good all round well stocked playroom.
For my 2nd, I have barely bought anything but she has shed loads to play with. She did of course get plenty at her first Xmas from family, and has plenty of her "own" things like dollys and stuff that people like to buy little girls but she also reaps the benefits of all the unisex toys I bought first up.
I have bought a couple of extras for her, one of those walker type things (but that you cant walk in) where you put them in a big pair of pants type thing with a table around them with toys attached to it (really can't remember what its called - activity station?). That's partly to incarcerate the little madam when she keeps chewing on cables and I need to leave the room to do something else.
If I carried on buying new things every month right throughout my son's existence and then for my daughter as well, however, we'd have far too much stuff.
Oh, and I dont mind having a cull now and then either. Jigsaws with missing bits, happy meal toys (not that we ever go to McDonalds, of course) and just general tat that doesn't work anymore goes either in the bin or to charity.