This is of course wholly dependent on the size of the website in question. However, we can answer a simpler question of ‘how long does the holding page’ take to do?
This would be a matter of hours. If we set up a hosting account for you and point a new domain over, this can be in just a few hours.
If we are pointing your existing domain to our server, it can be less, if your emails are staying put as (if we may get technical), we can do it by just updating your A Records.
A Coming Soon page can be just a few hours.