I've had an investigation for a secret clearance open since August 2016. How long does it normally take to be granted one?

It's entirely relative to you, your conduct, your background, and your connections. It can last one month to one year. The big thing that you need to know is that there are two parts: investigation and accreditation. Investigation I out of everyone's hands. Your commander can request an expedite on your accreditation at their discretion if they deem you mission essential.

depending on your past and what the extent of the background check it can take as much as 6-8 months...
TS is not much of a problem and in Most cases an interim clearance can be issued upon the initial investigation... but if it a BI of SCI they take a lot longer and they are going to go thru every thing you have done ot thought about doing and who and what type of friends you have

Lots of factors play into it. The location of all the people you used as recommendations plays a part in it. If they are all in different locations, it will take a longer time. If it is your first time, it takes longer as the initial investigation takes longer as opposed to a five or ten year reaccreditation.