There are three hardware series in the ScreenOS line: SSG; ISG; NS. So far only the SSG5 model in all these lines has been announced with an end of sale (January) and end of support (2020) dates.
http://www.juniper.net/support/eol/ns_hw.html
I would assume that as the Junos SRX series firewalls attain feature parity with the ScreenOS line and provide an easier migration path for the larger scale products they will also see end of sale dates posted.
As a general matter Junier gives us about six months notice for end of sale and gives 4-5 years after end of sale before support can no longer be purchased.