You are going to get answers everywhere from 1 day per park to weeks and you can't see it all!
There are people who "do"
Disneyland in a single day...yes, it can be done quite easily if you focus only on the rides and move quickly from one to another. But this is your first trip to Disneyland. So I highly recommend that you plan enough time to truly enjoy all that Disneyland offers that any other non-Disney amusement park does not.....the little nuances, the gorgeous detail in their buildings and landscaping...the fabulous shows on the schedule...and the often even more fabulous impromptu character interactions that happen all over the park, usually unscheduled, but ALWAYS worth stopping to watch.
If your trip is 2011, then there will be the new Little Mermaid and the updated Star Tours to not miss....and because they're new (opening June 3) the lines for them will be long. If your trip is 2012, by then Little Mermaid and Star Tours will be a little easier to get through the lines, but all the new Carsland stuff will be newly opened and of course you won't want to miss those either!
My family drives over from Arizona about every other month. The shortest trip we've ever had was 4 days....and even though we've been there hundreds and hundreds of days, it wasn't enough. But it was all we could squeeze into our schedule. Our norm is to arrive Saturday and leave the following Friday...so 6 days. Our favorite is when we can spend 2 weeks or more doing mostly Disney but occasionally venturing off to other Southern California fun.
Even though you won't have a car do not feel that you have to limit yourself to staying in a Disney hotel. There are MANY "Good Neighbor" hotels that are within easy walking distance....and others that are a little further away but offer shuttle service. We stay at Extended Stay America on Clementine because it's clean, cheap, has a kitchen so we can save a few dollars (and calories) by eating a good breakfast before we go, or do dinner at the room occasionally. Although we do have our car, we leave it in the hotel lot and walk across the street to the Toy Story parking and hop on Disney's bus for a quick ride to the parks. I mention this choices because the Disney hotels are rather expensive and often people limit their trip because of the high cost of the Disney hotels, not realizing that hotels at a fraction of the cost are a few steps further away. This is especially important to mention to folks who have been to Walt Disney World in Florida, where "off site" means miles away and little transportation in sight! If you can afford it, definitely stay on site because the hotels are gorgeous, there are some cool perks, and of course, it's Disney! But if you're more interested in the fun of the parks then stay at an equally wonderful hotel nearby and therefore afford more days in the park.
Also, realize that the cost of days in the parks goes down SUBSTANTIALLY when you stay longer. 1 or 2 days is quite expensive, but by the time you go from 4 to 5 days it is literally about the cost of lunch at your hometown fast food.
and I'm wondering how many days you would suggest spending on-site. I won't have access to a car so we'll be on property for the whole trip.
Just to clarify...I'm looking at the summer of 2012. We're heading to WDW in Florida this summer.
