Snakes have "indeterminate growth," meaning that they continue to grow throughout their lifetime. However, their growth rate significantly slows down after they reach reproductive age, and continues to decline each year afterward, for as long as they live. That's because they're devoting more energy/resources toward reproduction and less toward their own growth. Assuming you won't brumate her during her first winter, you probably won't notice any more growth after she's 3-4 years old, but she probably won't reach 1.5m. That's a really big garter!