No need for a timer. I used to use the clock in my classroom. They got ten mins then the bums' rush.
If it was an issue where they needed more time they were welcome to come in and I'd sit down and have a proper discussion with them, any time, and I made that plain.
Hate to break OP's illusions, but many schools I worked in the staff would have a competition (ie: race) and the winner was the one to get through their parents the fastest. I always won. I can be pretty determined though and am good at shutting things down.
Anything vital, and parents' evening's not the time or place, anyway. I'd much rather someone got my undivided attention even if I had to stay after school or give up a lunch hour for it.