In the US, if I were appointed to a tenure-track position, after how
many years would it be socially acceptable for me to leave? (I'd like
to go to this place for 1 or 2 years but not more - would that be
unacceptable?). The post would be in Psychology, and I'd be applying
from the UK so I'd want to go home after a year or two.

There are various post-doctoral positions available in the United
Staes.  Most of them are specific appointments for a year or 2 years
term.  If you choose to embark on a tenure-track position, it normally
takes about 7 years to get tenure.  You have the right to leave at any
time (and the employer has the right to let you go as well) before
tenure.  Tenure is for your protection, you can still leave after
tenure, but it's much harder for the institution to fire you after you
are tenured.  A university program might not want to hire you if they
know for sure that you plan to leave after a couple of years, but it's
not uncommon for new employees to leave if they feel that the place is
not a good fit.