I think BlueEagle is right on !
SOP's should dictate when and where you put the gear down and this will depend on your aircraft type and a number of other factors stated in the SOP's.
I have found that for light twins generaly on glideslope intercept or just before ie half dot below works well as most light piston twins have a gear extension speed down towards the cat B approach speed ( 120-180kts ) so it helps to get you in the groove as well as helping to innitiate the descent and get you slowed up.
If your not in a SOP type environment ie private pilot then it is up to you.
It will all depend on where you are how profficient you are and who is up your back side as to how fast you can or want to go down the slide.
I would also add in that it should definately be out at the outer marker at the latest so you can use the outer marker as a final check point and not have to run any checks from that point on except for the landing clearance checks and of course PUF ( pitch undercariage flap )prior to landing !!
If you use the outer marker or FAF as your get everything done by point you should be in a good position to face the nasty end of the approach.
Hope this helps ?