Twenty to thirty minutes on a sedan size car that's never been clayed before? In my view there are lots of variables in many of these responses.
1. How old is the car?
2. Does it live outside 24/7
3. Clayed before...yes/no.
4. How well the car has been maintained.
5. How much fall-out/contamination is on the car.
And it's likely I'm missing a few things.
I'm as meticulous as I want to be. More often than not, I err on the side of being too meticulous (not my words, rather other peoples comments). My answer to the question posed is that it takes as long as it takes. Regardless whether or not I do it for a hobby or I decide to go full-time. I refuse to lower my standards for a few extra dollars. Besides, at the point time would become a factor the additional time will be factored in the quote to the customer. The baggie test gives me an idea of what I'm up against.
I clay everything...paint, glass, wheels, painted trim, rubber trim. It's not uncommon for me to probably spend 20 minutes or more total, just kneading the clay from time to time.