I disagree with it being a 1 day park, to see and do most things, you need, to get everything done, it is 3 days. (That is every ride, every show, large and small, and then time to shop, check out the themeing, and eating at the main eateries on a GA ticket.
Now, the biggest issue are shows, and trying to schedule them into your plan, since they are on set times, and with a GA you need to be in line 30 minutes to an hour for most on the list to get a half-way decent seat.
Now, if you have been to Florida, you might want to skip some attractions, as they are copies of Florida for the majority of the ride.
Shrek 4-D and Minion Mayhem are the same, the Simpson is close enough and the film is the same. On the lower lot, Revenge of the Mummy is a lot different, Jurassic Park less so, but still both worth riding again. So now you are down to a two day park.
My recommendation for a first time visitor, go two days, the ticket media will make it about the same price and use a FOTL one day (Upgrade inside the park), unless you have a large budget, then it would be a VIP experience on the first day, and upgrade to a low cost AP for the second day, since you get credit for a GA ticket purchase in regards to upgrade a VIP.
One day one, focus on the FOTL, and do all the main rides and shows that you can work in.
Then on day 2, take it easy, ride things you want to re-ride, and finish up the show schedule, do your shopping and eat at Three Broomsticks, and do the Wand Shop Experience.
As a local, I would do a VIP Experience every other year, and then upgrade to a VIP at the end of the day. Let the AP expire, and then decide if you want to do the VIP again, or just get a new AP..
Of course, everyone travels differently.
As for the other parks, I would say that if you want to ride all the major Roller Coasters, SF Magic Mountain is a 2 day with a GA, sometimes 3 during a busy summer. With a Higher level Flash Pass, it goes down to 2.
Knott's for most is a full 1 day, if you want to spend a lot of time doing things like Ghost Toen Live, noww you are at 2 days.
LEGOLAND can get to be 3 days with all three parks, sometimes 2 (One Day focus on the Main park, the second day split between the Aquarium and Water Park.
SeaWorld is a Full Day.
Disneyland Resort is 3 to 5 days, depending on how many other Disney Parks you have visited already.