How long does it take to shoot a headshot is like asking how long is a piece of string? It all depends on who it is.
I have had Directors take one shot and say, "That's it. I look amazing. No retouching necessary".
I have had a make-up artist with a medium OCD issue take up to 40 minutes and choosing from over 50 shots. Retouching was eventually done on Facetime as she had to see the changes in realtime.
Travel to and from your business. Parking and setting up of mobile studio, which includes 2-3 lights, a backdrop of 5 ft wide. About 10 ft is needed coming "away" from the backdrop.
Images are put on a gallery RAW. They have no retouching. The client can then email their choices over and changes.
make me thin
make me fat
make me young
make me wrinkly OR
I am perfect the way I am
It is usually 10 minutes per person so perhaps 20-25 for 1/2 a day
There is usually processing time involved and retouching with communication with each client.
Eventually, the hi-res images are uploaded for your use.