I know what you guys are saying but I got a feeling that it's going to be mid game, in Act 2.
Because Mass Effect 2 has a "point of no return" as well which decides if any of your team member lives, or if any of Normandy's crew lives as well. You had to do all side quests, loyalty mission, etc. to get the good ending. Furthermore, post game in ME2 dropped you off after the final mission, not right before (to try and get multiple endings), so it did not matter with any of their DLC. Mass Effect 3 also has a forced urgency as it would be a matter of weeks before the Reapers wiped out all the major powers and started the harvesting process, but they have not 1, but 3 mid game DLC's that expanded on Liara's Shadow Broker, Aria's taking back of Omega, and a final DLC (that was actually the cheeriest DLC from ME3) that took place in the Citadel.
Finally, Final Fantasy XV had really sad endings (Noctis dies no matter what), but it had like 5 mid game/prerequel DLC that expanded on the lore. Did not matter that there are forced urgency either.
So therefore I think V's ending will not change, DLC will not be post game, and it will be mid game. The fact that V has 3 weeks to live doesn't matter, as if you actually did all the side quests (many of which required you to wait 24hrs of game time, if not more, to activate), you'd actually need 6 months(!!!) of game time to actually finish. Relic chip will not start to kill V until he does the point of no return anyways, and besides if you took the Nomad ending, it is almost as if the relic chip did not exist at all, and that final mission took place over 2 days of game time! You didn't even notice the relic chip until you're jacking into mikoshi. Actually as far as I know Nomad ending took the most time to do, as V basically went to the badlands, then they mobilized the clans and had basically an all out war with Militech, and it took probably almost 2 days of game time before V jacked into mikoshi, yet during the whole time it was almost as though V wasn't dying. If you chose Johnny to control V then the screen is all red and messed up the whole time, even post game (I do not know if this is a bug).
So I think a mid game DLC is far more likely, to explore the other areas of Night City, and possibly the crystal palace. I think anything post game will likely be in a sequel where V fades into legend, or it could be like Mass Effect where you load a previous save to make your lifepath and choices stick (except they will do little except a couple of dialogues and what NPC appears in the second game).
Finally, CDPR said they are NOT changing the story, and it makes zero sense to if you actually read up on the lore (this forum has a series of cards from the Cyberpunk 2020 game that really explained a lot of stuff). Johnny is NOT shoehorned in, and all the songs, like Chippin In, Will Not Fade Away, etc. are all in there! CDPR isn't making shit up. Also when the game credit rolls around, after all the calls to V, it plays a different version of Will Not Fade Away, and look up the lyric of that song and it tells you everything you need to know about how the story is supposed to go. Especially when it says "I have lost everything, we have to pay the price".