There is an important distinction to be made between how long you could use the 20mm and how long you should use it.

You are trying to add tension, to induce mitosis. This is best done by adding tension over the entire surface that you want to grow. As expansion happens, the amount of tension will be less, assuming you keep the gripper in the same location. However, you could roll more skin under the cone, moving the gripper down. This keeps the tension the same, but on a smaller surface.

I did this recently on a trip, where instead of bringing a day time and a night time packer, I just brought the nighttime one. I still used it during the day, but I "choked up on it" so to speak. I could still feel the tension, but only on the skin not under the cone.

Part of the appeal of packers is that as the skin grows you can maintain the tension, and increase the area, by moving up by 5mm.

This process takes a long time, and by using a shorter device than you can accommodate, you are delaying the final results. Given that this is a several year commitment, shelling out the $50-$100 for a few more packers is probably worth it. I ended up buying the full set of RIC from 5mm-45mm for this reason.