Ipfs doesn't provide a storage service as such, so:

> what is the durability of the files I store there?

You don't get the files distributed automatically. They're stored permanently only on clients which pinned them. (Either via user action, or because it's a storage service)

> How much can I store?

As much as you want. But you don't get mirrors by default.

> What is the availability of the files stored? What about latency?

Depends which nodes copy / cache your files.