I'm having the same issue on my 2015 Retina iMac. The install gets stuck after about 3-5 minutes. Always the same steps:
1) Reboot
2) "approximately 32 minutes remaining"
3) "approximately 31 minutes remaining"
4) "estimating remaining time"...
Then the progress bar remains unchanged overnight. Leaving it today all day and will leave it overnight too - so 24 hours.
Have tried rebooting while holding down"
"COMMAND+OPTION+P+R"
"COMMAND+R"
"COMMAND+D"
"COMMAND+R"
"COMMAND+D"
"OPTION+R"
"OPTION+D"
"OPTION+T"
"SHIFT+T"
"SHITF"
None of these have any impact on the installation process, it just boots to apple logo and process bar and gets hung up...
Have friend booting from a bootable external drive as well.
This is maddening, any help would be welcome.
Thanks.