for how long does the smooth feeling lasts for you?
for me, it depends. 10 up to 14 hours.

What you're describing is the difference between the shaved head and the bald head. If you have significant hair that you shave then, as with your face, it's just a matter of time before you have a version of cranial five o'clock shadow. If one is lucky, and tall, as I am, then few notice it. Also being old has helped as the chestnut is now gray or white. As with many men, I'd love the smooth feeling all day and all night but you can't fool Mother Nature.

If I shave my head at 6pm, I would usually feel hair growing through my scalp by 10am the next morning, so that makes it about 16hours.

I shaved my head at 11.45 last night. I then woke up at 5am and it was already rough. Not rough enough that anyone but me could see it, but rough enough to be not as smooth as I want it to be, so it was shaved again. It's now 2.15pm here and it's rough again.

i shave in the shower in the morning about 5:30 , 6 o'clock then reshave at about 9  i'm soft and smooth all day to the next day

reshave?
does that mean you shave again after like 4 hours after you just shaved?
if yes, that does not "kill" ur scalp?:O

For me I'd say about eight hours though the scalp "looks" clean for two or three days. I'm not an everyday shaver, BTW

About six hours for me.

A rough approximation is the same length of time as your face  though the hair on top tends to be softer so it stays less apparent a bit longer