That’s happened to me with my daughter many times. I’m pretty fast doing her catheter, but back before she got her Mitroffanoff, it did take a while to transfer to/from toilet.
(Mitroffanoff is a stoma on her abdomen that connects to her bladder that I catheterize her thru).
If someone knocks, I do yell nicely thru the door to say I am in with a disabled person and give an estimate of how long. I know they can hear me because I can hear them and they frequently reply back to thank me.
We got Security called once because a couple with a baby wanted us out. At the point thry knocked, we had just gotten in and I said it would be about 15 minutes more. The lady told the man that was ridiculous and told him to get Security.
A few minutes later, the man arrived back with Security and the woman said I had been in the ‘family‘ bathroom a long time and refused to leave so she could use it the changing table to change her baby. The Security Guy knocked on the door and asked if I needed assistance - I explained I had told them I was in there with a disabled person who I needed to physically assist and would be a few minutes more. The woman still wanted Security to force me out so she could use the room. Security told me to take whatever time I needed and suggested they use the changing table in the Men’s or Ladies room. She refused.
When I came out with my daughter, they were still there with about a 6 month old child in an enormous single stroller. They glared at me didn’t help me hold the door or anything as they pushed into the restroom.
I had to laugh at them because that was the tiniest Companion Restroom I’ve ever been in (it was by the exit of Pirates and has since been replaced).
It was just barely big enough for me to manouver my daughter on and off the toilet. It was so small that the back and one side of the wheelchair were touching walls and I had to move carefully to have find places to fit my feet. In fact, one time we used it, my older daughter came in with us and had to leave because there was no room for 2 people and a wheelchair (and none of us are big)
I don’t know how the 2 adults even fit in there with the stroller. And the best thing was that restroom had no changing table.