It takes some people longer than others. To be honest, I didn’t spend a lot of time on the leadpipe. I get it’s benefits and use it occasionally, but it’s not the same thing as playing the horn–playing the horn is easier.

The key thing is to make sure you’re engaging the corners as you start releasing air into the leadpipe/mouthpiece then slightly tighten the corners inward towards the center of the mouthpiece / air column. The pitch should change. It takes a bit more on the leadpipe as the slots are farther apart. On the horn, it’s a subtler, smaller change to change pitch.

Hope that helps.