I'm not certain whether or not this is the correct parameter but the following UCR variable does sound like what you're looking for:
Interval in seconds for the automatic machine password rotation
performed by Samba. If set to 0, the rotation is disabled.
It gets translated into Samba's "machine password timeout" which the man page describes as follows:
[code]machine password timeout (G)
If a Samba server is a member of a Windows NT Domain (see the
security = domain parameter) then periodically a running smbd
process will try and change the MACHINE ACCOUNT PASSWORD stored in
the TDB called private/secrets.tdb. This parameter specifies how
often this password will be changed, in seconds. The default is
one week (expressed in seconds), the same as a Windows NT Domain
See also smbpasswd(8), and the security = domain parameter.
Default: machine password timeout = 604800[/code]