Typically devs don't get final Dev kits until like 6 months before launch. PS4 final dev kits were sent out around may 2013. WiiU was may with the console releasing about 6 months later. So if the console is due out in say March 2020 then final kits would be main the rounds around september 2019.
The devs would get target briefs though or even non finalized dev kits which are pretty much specialized PCs.
Ok that makes sense now
Rules of thumb I would say to you that about 2 years before the start of new gen (that devs would know from historical data) they will also using historical data suppose a system they will dev for that new game they want to release on next gen.
And will dev with that target, anywhere between 1-2 years before release of the system the devs making launch window content will have ballpark estimatives of the system. But only 6 months before release they would get a real dev kit... sometimes they can even only receive a finished dev kit on launch if they aren't 1st party.
Remember that when PS4 was revealed on E3 the devs were surprised it had 8 instead of 4 GB of RAM? and that was less than 6 months before launch.