It lasts for at least two weeks.

As for the number of letters, they don't send that many at once, haha. It's more likely to be at most ten letters/packets every so often. But it's frequent and he'd be bringing letters back from the facility so a lot of "back and forth" will happen throughout the day. I believe the job is very similar to being a gofer.

Though I bet they would use Sentinel to send larger packages, so he'd get double the pay for those.