In real life when we go to a full service restaurant it is 3-4 hours - so I find it pretty much the same with my Sims couples.
Perhaps a lot of people are too used to fast food places - where you are in and out in an hour - but in our house I don't really see the purpose of going to a fast food restaurant. I know the Seafood Restaurant hubby and I like - we get seated about 6 o'clock and if we skip dessert which we normally do - we get out about 8:30 - 9:00 pm. That is just having a salad or chowder and the main dinner.
My Sims couples run about the same now. I do recall when it first came out though being a nightmare and often taking 8 or more hours with usually at least one Sim hardly eating because they didn't stay seated and had to talk to every one in the building. I remember wishing for super glue to glue them to their chair. I also learned to stop putting bars in restaurants - they were the bane of my existence as trying to keep them seated with a bar in the place was near impossible.
I do think the game could use a few fast food restaurants though that actually were for fast food and not just look like fast food restaurants - so that simmers had a choice of a full service dining out or a quick grab and run place with no socializing.
What I do now is just use the food carts for fast food and keep the restaurants for dates mostly.