EPA estimates 3 Billion gallons of waste cooking oil generated each year (see
Learn About Biodiesel | Region 9: Biodiesel | US EPA). While it seems like a lot, some of it is sent to rendering plants (couldn't find any reference to how much, but I'd guess over 50%) and some is simply unusable for biodiesel because it is over-used by the restaurants. I'll guess at 40% available for conversion to biodiesel, which is only 1.2 Billion gallons. (Actually, I don't think it's a 1:1 conversion from WVO to biodiesel... but I think it's pretty close.) This assumes that every available bit of WVO can be gathered and converted, which I'm sure is also not realistic.
Granted, the stuff going to the rendering plants could be used for biodiesel, but I don't believe they'd give up on it easily... they receive money both for the pickup of the oil and for the subsequent product they produce.
While I definitely like reducing our reliance on dino-fuels, WVO conversion to biodiesel will reduce overall dino-diesel use by 5%
at best, while 1% or less is more likely (2009 diesel consumption was ~43 Billion gallons according to the Energy Information Administration, see
U.S. Product Supplied for Crude Oil and Petroleum Products).