It seems to be more than half an hour? Does it vary?
According to FAQ:
Reputation here is entirely derived from the main website; your reputation is the same here as it is there, synchronized hourly. Votes here do not affect your reputation in any way. However, you can earn unique badges here on the meta site.
I've learned about this from this answer on meta.SO.
EDIT: It seems that the above link to faq has been changed in the meantime. The same information is now contained in What is "meta".