As others said, there aren't any real regs to fall back on, however,
OTS also recommended at least 5 consecutive days as a policy
standard.
To be frank, in my humble opinion, other than a teller, I don't know how they can catch someone doing something amiss, who's out for just one or two weeks.
We did bust someone who was out on medical leave (6 weeks), but it
was purely an accident.