We typically get 4 day park hoppers for our family of 4 (now 5, but we haven't made the trip with our baby in tow, yet). We could see the parks in less time than that, but the 4 day hoppers allow us time to have a mid-day rest. It really makes all the difference in the world for us. I assumed for our last trip, with one child being almost 9, and one being 5, we wouldn't need the mid-day nap, but boy, was I surprised when my 5 year old ASKED for one! We would typically arrive around 9:30 ish, which was pretty much rope drop, because it was January, and later opening times, then we'd be ready for a break by 2:00 pm, head back to the hotel for a nap, a swim, and some pizza, and then come back into the park around 5 or 5:30. This worked really well, because we didn't miss any of the incredible night time entertainment, and were all well rested enough to really enjoy it. Besides WOC and Fantasmic, there's a Pixar parade which is incredible at DCA at dusk, and I've heard really incredible things about the new PTN parade, and Disneyland Forever Fireworks, though we haven't seen them yet.
As far as Park Hoppers go, I would reconsider that as well. Park Hopping at Disneyland is a lot different (and more convenient) than park hopping at WDW. The two resorts are literally right next door to each other, and it's almost more like one park with an atrium in the middle (albeit totally separate theming). There's no hassle of transportation, and waiting for trams, etc. It's actually really nice to be able to be bop back and forth. Especially if you are able to see WOC in the evening at DCA. It's definitely an increase in cost, but for me, it's worth the increase to have options, and the style of touring we do is much like that.
At WDW, we prefer to have at LEAST a full 7 days in the park, and we utilize the hoppers there, as well, mostly because, as I said, I prefer options. If I had to choose, though I could easily survive without hoppers at WDW, but would never go without hoppers at DL.
I don't think you can really have "too many days." When we went to DL the first time, having only been to WDW, I thought even 2 full days would be overkill (and this was before the DCA expansion), size wise, it was so much smaller, I just knew we'd see it all in two days. But, honestly, it just wasn't even close to enough. And that was with an 18 month old only! The 4 days work well for us. We live about 10 hours away, and drive. We usually arrive around 2:00 pm Cali time, and so our first day is a 3:00-close kind of day. But the next three are full days. And while that's a good number for us, we could easily spend another day (or even 2) to fit everything in. We usually go during slower times too, if you're going during a busier season, I think you will want more than 1.5 days at DL. No matter what you decide, you'll have an incredible time, though, have fun!