I sort of a restaurant buff and my best restaurant experiences is when the restaurant is brand new and the owner is looking to build a clientele. Once the restaurant is full of paying clients, things start to change as the owner now focuses on profit rather than gaining market share. The waitstaff are asked to work 10 tabletops instead of 6. You now have to ask for condiments instead of having them on the table already. The portion sizes are a little bit smaller. The vegetable side-dishes now appear to have been microwaved from a frozen state instead of being prepared fresh. It's now time to take that restaurant off your list.
You're exactly right. In our area it usually takes only 3 - 9 months. Often the chef has a falling out with the owner at about the three month mark. Then Ernesto takes over the kitchen and that's it. Maybe one in ten new restaurants makes it a full year, because rents around here are excessive.
Las Vegas buffets follow the same principle. Go to a new one right after it opens - because the accountants start screaming at about the three month mark, and by nine months the food becomes inedible. :)