True, however we can still set expectation even if there is no way to enforce it. This would be more of a social norm than a hard requirement, but it does help to know that if you are e.g. pushing an entire board into coreboot, that to keep it in tree you are expected to do <x> amount of work over the coming years. That in turn may (or may not) influence the decisions to try to do a port / push the board upstream in the first place, or in a corporate setting potentially get the additional resources allocated to avoid merge / remove / merge cycles that are more expensive than merge / maintain.
I mean, I think I'm trying to say that the we should put this as "if you (or someone else who happens to be interested enough in your platform) don't keep doing this, this and this, your platform will get deleted". I don't think we can really put a timeframe to it either... the board will just stay in the tree for as long as it gets sufficiently maintained. (We can point out that maintenance burden always tends to rise with age, though.) If an SoC doesn't get maintained at all I'd expect that it may on average survive somewhere around 3-5 years before people get too fed up with it, but you can't really put a hard number on that... it always depends on what sort of core API changes we end up making and how hard it is to pull that particular SoC along (of course following best practices and trying to stay within the common frameworks also naturally increases a platform's chance of surviving longer).