Whether you bought a standing desk to improve your posture or to avoid the health pitfalls of sitting all day, using one isn’t quite as simple as you might think. In fact, you can even stand too long at a standing desk.

So how long should you stand at a standing desk? 15-20 minutes per hour should do the trick.

Dr. Linda Miller, a certified ergonomist, occupational therapist, and CEO of EWI Works, spoke with Wirecutter explaining that a standing desk is designed to encourage movement—not just standing. Standing, on its own, isn’t exercise. Instead, standing desks incorporate movement into your day through the act of getting up and out of your chair, shuffling your feet as you stand, raising your legs, and even fidgeting. In fact, prolonged standing can actually be bad for you.

Standing for an hour or two at a time can increase bodily discomfort, elevate blood pressure, and encourage varicose veins. That’s why Miller recommends 15-20 minutes of standing per hour, but if you’re used to sitting all day, don’t try this all at once.

Miller recommended starting with just five minutes per hour. Then, once you’re used to that, work up to 10 minutes, and then move up to 15 minutes. Increase your time according to how your body feels, and eventually, you’ll hit the benchmark.

If you recently invested in a standing desk or have one you tend to not use, keeping your standing time to 15-20 minutes and working your way up to that time frame might just have you putting that desk to good use.