It depends on where you live. In the US, different states have different laws regarding this, but I believe most say something about how long you need to wait. Check with a friendly local LEO for advice. Also, depending on the type of sensor, it might not even be a weight issue. Most sensors are actually looking for a strong magnetic field. If you can see the sensor lines in the road, I've heard that putting your kickstand down one of them will do the trick. YMMV.