I didn't notice it in the ipad Cell Menu, but the default is AUTO-RENEW. this means your plan will renew the 250mb data balance (and charge you) automatically every 30 days.
if you use all 250mb in the first week, you will have no data (and no more charges) til the next 30 day cycle starts. (You can reup the plan early and pay another $15 manually.)
if you use, say 200mb in 30 days, on day 31 (new month) you get a charge of $15 and a new beginning balance of 250mb for 30 more days time. it does not roll over.
that's how I read the statements they've emailed me.
EDIT - I'm on the 250mb plan, it ends June25. I changed my plan yesterday for the Unlimited plan. The Unlimited plan will start June26 ...OR... when I run out of 250mb.
oh! AT&T, they think of everything.
and Wow, I was just calling ATT to make sure my Unlimited plan was set up right and they are closed. It's 830pm local time, Sunday. oh well.