i had to return my 360 because of the 3 flashing red lights about 2 months ago. i had that xbox since the release day. anyways, it was pretty fast. the email system wasnt very helpful, because i would say "i read the online help guide, and checked the power supply"...etc, and they would email me back telling me to make sure to "check the power supply."
they did that two times i believe. other than that, it was pretty fast i THINK. took about 2 weeks to get a new xbox.
to answer ur question (just realized that i didnt before), they usually got back to me via email in a business day or two. phone wait times can be a pain, but they werent horrible, maybe like 15-30 mins max. actually, i usually get right through i think, only had to wait a few times.