How long will an index take to create?

Actually - it depends on many things like

• hardware you use
• wideness of index
• current load
• ...

If index is relatively wide - it can take from minutes to hours.

but

if you have your DB on Enterprise Edition of Sql Server, than you can choose to create an index WITH ONLINE=ON - it will take a little bit more time and space to complete, but will not stop other processing with the table involved

Yes, it depends on a many factors and there's no way to really know unless you try.

I just built an index a single column of type BigInt over 200+ million rows however and it took a hairy 3 minutes.

Sure, my hardware is different from yours, my database version is different from yours etc. etc. but now you have a real world example to give you a ballpark idea rather than somewhere between a picosecond and a googolplex of lifetimes of the Universe.