I am not sure what is currently going on with the Azure Guest OS Availability.
Currently 3.17 is the default active one, and 3.16 is the non-default inactive one.
There are however two versions of each of 3.18 and 3.19 waiting in an inactive state.
The only source of dates for the expiry of these is:
3.16 is listed as expiring yesterday (Oct 10 2014) and 3.17 is due to expire on monday (Oct 13 2014).
3.18 has already expired and 3.19 has a TBD release date.
Given that I work with a product that we like to test on an os before we actually deploy it we are in a rather difficult position. We can deploy what we have to test but will at some undefined point we will need to move to the newly released versions and retest them.
This is not a very satisfactory poisition to be in.