I would say 4-6 days. We went 3 days last month (our first trip) and it just wasn't enough. We're going back for 5 more next month.
It's really not much more for extra days, beyond 3. Your flight/travel costs are fixed. The incremental cost for longer park tickets are minimal. The hotel and food are the biggest variables, and you can stay and eat cheap without sacrificing the quality of your vacation.
Be planning for next year, too. If you think you'll come back, consider upgrading to season passes and schedule next year's trip a week or two earlier than this year's.