There's a link to "Your Usage Dashboard" on the landing page of the Development Platform. On the Dashboard you'll find a complete list of all deployments, how often they've been called, and how many Cloud Credits they've used. You can also "un-deploy" any deployment (which essentially deactivates the link, you can re-activate it later). I'm not sure if you can permanently delete anything though... Also, the Dashboard can be a bit sluggish for me and sometimes parts of it refuse to build, but something useful usually happens eventually.
As for lifespan in general... I only have the free version of Development Platform which according to the pricing page has a "30 day limit for deployments". I'm not entirely sure what that means because I know I've been using deployments that were definitely much older than that. Maybe it's a rule that's only enforced when resources demand it.
