I'm an existing Sky Customer. I ordered Sky + on 20 Feb and was told over the phone that it could be a few weeks to wait, and depended on the amount of installations in my local area. However, I got an email the following Monday offering to do it by the end of February. However, I chose to wait until 8 March as that is the day after my new HDTV is being delivered. You will probably have to wait a lot less than the three months Sky quoted, but there may be regional differences (eg London may be busier than the regions I guess).
Separate question - when Sky come out to do the "installation" can I ask them to change the cable wire and LNB Quad. I recently moved in to the property and am getting a weak signal and guess that this is the old cable, as I have had the dish properly aligned.