Originally, blob leases were simply 60 seconds. Starting with the 2012-02-12 version, blob lease durations can be between 15-60 seconds, or infinite. You can read this blog post for more details around all the updates in the 2012-02-12 update.
If you want to see the actual REST API details behind the SDK, it's documented here. In this page, you'll see that an infinite lease duration is specified as -1
(which is slightly different from, say, the .net SDK call, which uses null
to signify infinite lease).