HELP PRAY EXTRA gives a lot of good detail about this. Particularly point #2.

2. Now, you've sent up a pray. Wait for a response. You can go to sleep, we can send tells to sleeping people. If no one responds in a few minutes, then send up a second pray, but DON'T send up multiple prays at once, we will not like it. If still no one responds, then you can send up prays every couple of minutes as long as you want, until someone responds. If you have a very specific pray, though, then just write that immortal(s) a note about it. Yes, we do read our notes. If you think it's really urgent, then continue praying once every five minutes or so (if you feel like it) until someone logs on, but notes are a better way to catch someone if no one is on initially.

Also, an excerpt from HELP EMPOWERMENT:

In all of these instances, you must remember to be respectful of the gods. If you pray constantly (more frequently than every 15 minutes or so), you are only irritating the gods who are currently present. If you treat empowerment as if it is your right, you will also soon learn that empowerment must be earned. Finally, if you have no response from the god you are trying to gain audience with, post them a note about yourself. No other god will be able to answer your questions about empowerment.



Overall, I've generally gone with the "no more than once every 15 minutes" rule. The Immortals have ways to look for who's praying for them, particularly if you're including your name in the pray. I've found these days, the Immortals do tend to be pretty awesome about finding you if you need empowerment. Zulg's post is pretty much about typing "pray XYZABC" and hitting enter 20 times in a row.