IIRC for most US soldiers, the typical tour of duty in Vietnam was 13 months, which included a 30 day R & R. Was there a maximum amount of duty allowed? Could one have done 4 years straight (voluntarily), say from 66 to 70 or would they have been assigned elsewhere even if they had wanted to remain in Vietnam?
It was 13 months for Marines. Army, Air Force, and Navy did 12 months.
R&R was 7 days, typically. Though I think under some circumstances you could get an extension, but that was rare.
Toward the end of your tour you could extend it for 6 months, which a surprisingly large number of men did. I’m not sure if you could do a second extension or not. For various reasons (it didn’t look good, I’ve been told, being the main one) they wanted you to go home for at least a while. Once home you were, supposedly, assured of being there 6 months before being sent back. But a lot of men got home, found they didn’t like it and requested to go back fairly quickly. I knew a guy that I served with that did almost 3 years straight. He had a couple of short stints in the US mixed in.