Generally, it is not THAT much different. They basically just redid the center (Pleasure Island) and made it a large "themed" outdoor mall. The two other sections (Marketplace and West Side) are basically the same.
As for how much time to allow? It just depends what you want to do there. My last two trips there it was under massive construction.
It's fun to walk around, grab a bite to eat and just take it in during the evening.
I like to go to the Marketplace to get a "taste" of Disney on my arrival day for just a few hours before we hit the parks the following day. You can buy some souvenirs, new Magic Bands, pins and gather your park maps. The kids love the
Lego Store and it has great Disney-themed lego sculptures inside and out. Find a place to eat like T-Rex or Wolfgang Puck express and head back to the hotel. Some times I've spent a few hours there and never ventured outside the Marketplace.
There are other fun things to do there too .. boat rides, balloon rides, etc.
I like to go there if I am on a Universal vacation to get a taste of Disney as well.
I personally can't imagine going there specifically to shop at the non-Disney stores or go see a movie or go Bowling. Those things seem for the locals - not for people on vacation.