If we are going ahead with this motion, I like Gabriel's suggestion that it should be witnesses who are used to this and are regularly invited. I don't know how we can phrase that. I can tentatively offer “witnesses who are regularly invited to this committee”, or maybe “senior officials and ministers”. I don't want to be part of a committee where the chair has to look at 30 seconds and then 30 seconds. I think it would be unfair to put that burden on them.
I would change the last sentence. Instead of saying “unless with the permission of the member who has the floor”, I would say, “when the member who has the floor asks for a short reply”. That would seem to be more.... If you ask a short question, you could ask the witness for a short reply. The chair will then know what's going on and be able to compute the time.
When we invite witnesses, I want to hear what they have to say. Just because I ask a 30-second question, I don't want to put a burden on the chair to interrupt the witness when I want a long answer, because they're there to provide their expertise.