Depends on three things...
How crowded the park is
The park hours/day of the week
And how much of the park you want to see.
To give you an idea, DH and I were able to do about 3/4 of
Disneyland in *one* day on a Saturday in May when the parks were fairly crowded (we did see waits of 45-minutes to an hour) without waiting for *any* ride for more than 20 minutes in line. But, we were there from park opening at early entry (I think that was something like 6:30am) to about 30 minutes before park closing at midnight. We did bottom-out around 6pm and just sit at Coke Corner for about 2 hours where we caught the parade and the fireworks before catching the second Fantasmic, but other than that we were go-go-go the entire day. You *do* need to know the park rather well to do this, though -- do some research before you go. Figure out which rides are likely to fill up fast, use Fastpass, take a break for a sit-down meal or an hour's nap mid-afternoon, and learn where everything is.
Doing all of that, you can indeed see most of the park, though definitely not all of it, in one day only.
Good luck, and let us know what you decide and how it works out if you go
-Rajah
cdrtrisal@aol.com
Rajah and VonDrake's Honeymoon
Rajah's Disneyland Crossword Puzzle