I use mine until I have to charge. Generally, I am going to be near a charger long enough for a full charge. I have one at work, 2 at home not including the sync cable (my MacBookPro will charge the iPad even with the screen on, unlike my old HP and other Window machines). I get 2 to 3 hours solid use a day, so I end up charging about every 4-5 days. With an hour commute each way on a bus everyday, There is 2 hours right there.
The only time I charge it when it is above 5% is when I know I will be out for an extended period of time or know I will be using the screen at full brightness for a decent period of time.
Right now, I am at 35% and dont plan on charging unless I really use it a lot tonight.