Not much to add from the other comments, I would suggest you try and get there early and miss the queue's.
2 hours is good, it's an amazing museum and so well put together, so enjoy.
I would say that 1.5-2 hours would be good, depending on your level of interest! There is a lot of text so it could take longer, but unless you're extremely interested in the subject I'd say the above is about right.
If you have fairly solid background knowledge about the Islamic faith and history, then one hour is enough, to appreciate the way they curate and put together the ideas and exhibits. If you are here to learn from (nearly) scratch, 2.5-3 hours would be good. That said, that's a lot to absorb. If you are a pick-and-choose type of museum visitor, then 1.5-2 hours is more than enough.
There's a lot of text displays to read so if you find yourself running out of eye power or patience, you can always take some photos and go home to pore over them.
Hope this helps. And enjoy the visit!
It's a big place. If you just run through it, perhaps 1 + is enough, if you want to concentrate, perhaps even 3 hours.
If you want to get a good understanding on the subject, if you are interested in history, then around 2.5 - 3 hours.
Get quick answers from Sharjah Museum of Islamic Civilization staff and past visitors.