Apple recommends a full discharge about once a month. From what I read, what's more important is making sure that you don't expose the iPad to temperature extremes.
According to Apple, the battery should have something like 80% efficiency after 1,000 full charge cycles. Even if you go through a full charge cycle every day (10 hrs of use), that's three years.
Edited to add that the rate of discharge will depend on what you are doing. I find that downloading videos is a battery drainer - I noticed that I would lose a percent every 4 min. when downloading, as compared to about a percent loss every 6 or 7 min when surfing, and about a percent loss every 8 min when watching videos.