How long can I expect for it to complete?
It depends on how old/slow a system you have. I run 1ghz with 512mb ram and the whole install from start to finish takes about 20 minutes.
I'm trying to dual boot windows and linux. I've set up the root folder of linux on an external HDD. If I boot the computer after installation will I have an option of which OS to choose?
This questions is a little harder to answer.
By choice, do you mean bois choice or boot loader choice?
Bois choice depends on your machine. Some bois will allow booting to an external HD, but won't asset so after grub passes control off to the kernel it hanges because the kernel hasn't loaded all the modules yet and it doesn't have the proper drivers to handle the USB drive. Other Bois seem to work fine even after passing off to grub and the kernel.
By choice, do you mean grub menu?
That will depend which mbr you install grub to. the internal or external HD. Once again you might have problems loading the system after grub passes it off to the kernel. If you put the grub on the internal HD mbr, and the root is on an external HD, that will create another problem No system will boot without the external HD present, because /boot is on the external HD.
If your BIOS supports it I'd install the grub boot loader to the external HD and hope it works. If not then there are other work arounds. But it would require partitioning the internal HD to put /boot on the internal HD.