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