webserver delivery: primary storage

Planning to deliver a webserver application (unfortunately) on a Win machine.

By the way, this might become a business application and I presented my plans
yesterday to the customer. Had to fight the 'why Lisp'-battle and am yet confident.

There arose some questions I couldn't answer. One of them was:

When all the webserver data is loaded directly into the Lisp process and the
data is a product catalog with thousands of products and several languages, the
limits of primary storage might become relevant.

Is there a rule of thumb how much data I can load into my Lisp process with
respect to size of primary storage? When will it be more appropriate to use an
external database?

jens