My boys were in 0000s for 3 or 4 weeks I think. Not very long, but they grew REALLY quickly (went from 50th to 90th percentiles by their 2 month check ups).
I would expect an average sized bub to be in them for about a month. Maybe get half a dozen little suits. Don't go overboard, it isn't hard to send someone to the shops to grab some more if you find you need them after bub is born.