aptitude update will only get new information about packages & updates to your installed packages.
The command to actually install updated packages is
aptitude safe-upgrade, (not
As for how often you should update, it is "at your discretion." While derivatives like Ubuntu have recommended settings, it really depends on your preferences - would you rather be up to date, or would you rather an update not break something until you are ready to deal with it?
On a desktop you use daily, a daily check is probably best - you want the updates the day they come in because you're using the machine regularly. On a server, I'd go for once a week or your regularly scheduled maintenance window. (If you don't have a maintenance window, now's a fine time to set one.)