1 day consists of 1440 minutes.

Yes. If you enter a large enough number there, it updates in those intervals.
If you update manually once with App Manager, the "clock" restarts, and the next auto-update will be after that time plus the specified interval.

If you are concerned about the data costs, enter an absolutely huge number there, and thus "disable" the auto-updates.
Then, if you're on a home wi-fi or something, you can press a button while you eat or work, or something, and the N900 will happily update itself manually, without bothering you.

BTW, there is a "Thanks!" button. :P