There's no hard and fast rule. Access has no specific record number limit; the
entire size of a database cannot exceed 2GByte, so if you're pushing that,
it's time to move. It's more often going to be issues of security, network
architecture, hot backups, etc. that drive the decision. DON'T expect that
moving to SQL will make your database faster or easier to maintain - it's
likely to be somewhat slower (but more scalable) and will require a good bit
of work.
See http://www.granite.ab.ca/access/sqlserverwhy.htm for Tony Toews' detailed
(and somewhat idiosyncratic) take on the question, or search the
microsoft.public.access.sqlupsizing newsgroup on Google Groups for a wide
array of opinions.
