Hi,

Just click on the transition symbol [A|B] again, ideally in timeline mode, and pick the "Set transition length" option from the menu that will pop up. Set the slide to the desired time and confirm the dialog.

You could also just use your mouse in timeline mode after picking the "door" transition and push the second object further to the left into the first object to make the transitional phase longer or move it to the right to shorten the overlapping section, therefore shortening the transition length.

I hope this helps.