Great Power contentions

[Pannonian]

Well, it will probably be based on verification. After demilitarization, it would be impossible for Ukraine to increase their combat capabilities without Russia knowing about it. That would include tight limits on foreign military involvement.
Most importantly for Russia, that would certainly include Ukraine being legally forbidden from allowing any NATO military equipment on its territory.

Such treaties are usually in effect as long as they are enforceable. So, we can probably expect it will for sure be in effect for at least a few decades, and after that it will depend on the balance of power in the region and the world.

Demilitarisation? Why on earth would anyone agree to demilitarisation after their neighbour had invaded them and taken chunks of their territory?