If you are in pull mode, the rate that your heartbeat is set at will affect this.
If your hearbeat is set at 30 minutes and ... and the machine's last heartbeat was at 10:15 AM and...
If you get a virus at 10: 30 AM, you are not going to get alerted until at least 10:45 AM.
at least that's my understanding..
edit:
nevermind, I just re-read your question, and I don't think I've provided the correct answer.