I have no idea, only a guess.
If you consider a "local" someone who lives in the five central Florida counties (Orange, Seminole, Osceola, Lake, Polk), you get 2.3 Million people. If you throw in the Tampa/St. Pete/Clearwater metro area you get another 2.7 Million people. That's a total of
5 Million "locals" that live within a 2 hour drive of WDW.
It's alot, but then you realize that the six counties of SoCal (Orange, Los Angeles, Ventura, San Diego, Riverside, San Bernardino) comprise
21 Million people within a 2 hour drive of Disneyland.
http://en.wikipedia.org/wiki/Southern_California
So there's a good idea at the power of locals at the two Disney properties.
There are about 8 Million people within a one hour drive of Legoland California, in the San Diego/Orange/Riverside counties. There are about 2 Million people within a one hour drive of Legoland in the Orange/Osceola/Polk counties.
Interestingly, there are 8 major theme parks in SoCal, and 8 major theme parks in Central Florida. So spoken broadly, the tourists are driving the parks in Central Florida, and the locals are driving the parks in SoCal, with a few exceptions. The parks in SoCal are spread out from Magic Mountain in north LA County to Sea World down in San Diego. The Central Florida parks are fairly close together, although Legoland will be noticeably separate from that Orlando core. You could also make the argument that Busch Gardens Tampa should be included in the tally.
SoCal Theme Parks
Six Flags Magic Mountain
Universal Studios Hollywood
Knott's Berry Farm
Disneyland
Disney's California Adventure
Legoland California
Sea World
San Diego Zoo/Wild Animal Park
Central Florida Theme Parks
Universal Studios Florida
Islands of Adventure
Sea World
Magic Kingdom
Epcot Center
Animal Kingdom
Disney's Hollywood Studios
Legoland Florida