I bought everything for the Sims 1 and 2, so I imagine I'll end up doing the same for the Sims 3--discounting the stuff from the store. As it's the only computer game I play, I don't feel too guilty over this. Mind you, this time I'm in the position of having a baby (although just the one), so I may yet decide that the limited gameplay time I have doesn't warrant every stuff pack. We'll see.
To agree with what other people have said, I expect the Sims 3 expansion packs will come out every six months.