Juniper does not list any entry at all on the hardware EOL pages until an end of sale announcement first comes out. So when there is no listing then you have no starting timeline at all for EOL.
Also notice that with the SRX and SSG models there are specific announcements for specific hardware versions, not the entire product line. So you need to confirm on your device which hardware version your are running of the device in question. Thus we see EOL timelines for some modes like SRX100B but still active sale and not announcement for SRX100H2.
As a general rule Juniperr announces end of sale on a specific device about 6 months in advance. Juniper will still allow you to purchase support contracts for between 3-5 years after this last sale date. There are adjustments in time as to when same day support ends earlier than next day support.