I do use it for work, but I prefer to ride my bicycle when the weather is nice--so it has lasted me a very long time. That said, the stipulations listed around our commuter plan have only stated the types of services it can be spent on, and not when they can be used.

I put $300 on a transit card two years ago from a commuter account. I use the train all the time and I still have $100 left on the card. I figure anything you would use anyway, you might as well do tax free if possible. That last $90-ish in train rides is essentially free.