Approximately how long does a Botox procedure to slim down the trapezius muscles take?

Perhaps because my job requires me to sit in front of a computer for long periods, my trapezius muscles in my shoulders and neck have become abnormally bulging. It makes my neck look short, and the fit of off-shoulder tops or summer clothes doesn't look good, so I'm planning to get trapezius Botox. Since I'm pressed for time after work on a weekday, I plan to stop by right away. I'd like to know in advance how long the entire procedure takes. Excluding the consultation and waiting time, does the actual procedure—lying on the bed to get the injection—simply take a few minutes? I would also appreciate it if you could share information regarding the downtime for shoulder stiffness!