Depending on Mac vintage, internet recovery is an option.
As to time to install, from my experience, not all USB sticks are created equal. For example, Lexar drives are really slow. Add in if you have an HDD, yes, might be installing, and yes, will take that long.
An iMac I just updated with an HDD took an hour, maybe a bit more via standard installer ofF the HDD.
I got the usb which I used always for the past 3 years, it never took that long.
This is a brand new machine, just bought it yesterday, it has no HDD but an SSD.
Yesterday evening I did the step with the receivers and install OS , he showed me Mojave’s as well and this was the one which took so long.
I have read about it, but I have not done it. I have a MacBook Pro 2015. But you now know where to start.
I read online also that I can easily use the recovery mode option which will then download the latest (Mojave) OS kn the machine without backup. I did this yesterday and needed to stop after three hours...