It depends. Whenever something changes, the whole Billboard text is re-evaluated, causing all scripts to run as well. So e.g. if you have the battery indicator there as well, all scripts are run whenever the battery indicator changes.

Apart from that, there's a system-aligned timer (meaning that it tries to align itself with other activity from the system to not wake up at unsuitable times) that has a minimum duration of 30 minutes and a maximum duration of 60 minutes. So at least it is refreshed every hour, but might be up to every 30 minutes.

If you have other things (e.g. with ContextKit stuff) then it refreshes according to these updates, and in addition (of there are no updates) again every 30-60 minutes.