How long does Bootcamp Take to install windows 10

It depends on your computer and its storage drive (HDD or flash storage/SSD), but a Windows installation may take from 20 minutes to 1 hour.

I'm not sure I understood your second question. Boot Camp lets you install and run Windows like you would do on any other computer, so there's a great advantage: you will get the best performance possible instead of what you would get from a virtual machine. Apart from that, OS X is enough for most users, so Boot Camp is only useful if there's any application you want to run that is only available for Windows. See -> Windows in BootCamp or Virtual Machine?

It takes no longer than it would take to install it on any other similarly equipped computer. Boot Camp doesn't install Windows. Boot Camp merely enables you to boot the computer using a Windows disc and install and run the software on the Mac's hardware. This assumes you did everything properly. Windows will run on a Mac just exactly as it would on a Windows laptop with all the same hardware. So, yes, it is useful to keep.