I don't know how things were then, but today "The oath is for an indeterminate period; no duration is specifically defined" [Wikipedia].
The bigger question is, when your name has become synonymous with personal honor, as Lee's was, whether your word should be as subject to changing circumstances as Lee's apparently was.
If Lee and other officers had made it clear that they weren't going to change their loyalties, perhaps war could have been avoided.
The oath, with regard to military service, is for as long as the commission or enlistment lasts. Beyond that is up to the person in question and is simply a matter of choice, not contractual obligation.
The bigger question is, when your name has become synonymous with personal honor, as Lee's was, whether your word should be as subject to changing circumstances as Lee's apparently was.
Since the inception of these United States and the Constitution that binds them, States have been pre-eminent. The Articles of Confederation attempted to make a cooperative of sorts, but there was no mutual benefit amongst the various States. This led to failure of the AoC. However, loyalty was ALWAYS to the State before the Federal government, which is why Lee chose Virginia. Were you to survey the divisions and regiments of the War Between the States, you would see an overwhelming amount of State flags and banners vice a single Stars an Stripes or Southern Cross.
If Lee and other officers had made it clear that they weren't going to change their loyalties, perhaps war could have been avoided.
Interesting supposition. There are probably a good many articles written about such a thesis. Might be worth you doing an article on and submitting to one of the many magazines dedicated to the WBtS.