The speed really depends on depends on what your trying to do. Trying to floor it will mostly cause wheel spin which is good for clearing the tire tread, but when you are starting from a stand still it really doesn't do much more than that. Some people get the feeling that when they floor it they are going fast and the momentum will let them push threw but if you don't have a running start that doesn't get you anywhere.
When the snow is really deep I usually have really good luck with this trick:
1. Lock it if you got it
2. Start slow going forward and stay moving slow
3. When/if you feel the car slowing gradually let off the gas
4. when you stop switch it into reverse and back up over your old tracks
5. Put it back in drive and do it all over again
This not only gives you some space for run up but also gives the snow you are pushing/compacting and building up under the car a place and chance to clear. Give it a try.
2007 SX4 JLX Pearl White - RRO Lifted, Sparco Drifted, 215/65/R16s, Trim/Racks Stripped...
2009 SX4 JLX Vivid Red - Lifted, Hitched, 215/65/R16 General Grabbers, Slotted Rotors, LEDs Everywhere, DMM HIDs, LED Tails, Rally Armour Mud Flaps, Black RR...