I don't think you need to use anything very scientific. Too high
shouldn't hurt, I don't think memory is allocated needlessly. The
cache timeout value is probably more performance worthwile.

But to calc, I would go with about the number of logins you get per 5
or 10min. I would think any logins over that period of time would probably be
the same user.

Quoting Frank Bonnet <f.bonnet@esiee.fr>:

Hello

Is there a way to calculate this parameter for my site which has approx 4000 users authenticating thru NIS ?

Thanks a lot