This is impossible to predict since everyone carries weight differently, however, since a lot of the fat cells in that area would have been removed, it is likely that it will be more difficult to put on weight there than it was prior to the procedure. So, if a 10 pound weight gain used to show up under your chin, it might take a 15+ pound weight gain to show up in your chin after having surgery. Liposuction doesn't mean you won't ever gain weight-- it's just that if you do, it is usually distributed more evenly vs. all going to your typical, hereditary areas.
All said, you will enjoy better and longer results if you keep your weight stable after having any kind of liposuction.