Once the text package is activiated, it will be renewed every month provided you have sufficient funds in your account. And yes, they use your activation date as your anniversary date, so if you activate on the 8th, it will be renewed every month on the 8th.
On the iPhone 2G you can check your phone usage under Settings/General/Usage/Call Time. There isn't a built in method of keeping track of the number of texts you use, but I purchased a cheap app from either Cydia or Rock called SMS Counter. Its just an icon that sits on your springboard with a red badge indicating the number of texts sent within a configurable period. Currently I have sent 68 texts since June 10th, so until I reach 100 texts in that one month period it doesn't make sense for me to get the text package. Handy little App!