how long should it take to install

install of what? If you talk about the installation of the Proxmox VE ISO, its just a few minutes.

But do not install on USB drive, not reliable enough.
 
its probably a slow USB memory stick - i agree with Tom - USB memory isnt reliable enough - i tried a similar approach with my systems originally and have reverted to standard hdds
 
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If you have limited space for HDD´s, just use a small but reliable SSD.