Is it worth it to have more that one tank of the same faction?
Well, as a player that has 3 tanks, one of each AC, at 50 on the same server for the same faction, I can say that it's worth it but much of that is due to my dislike of playing DPS and enjoyment of playing tanks. As such, it depends greatly on how much you enjoy tanking. If you do, you'll get a lot of mileage out of running multiple tanks, especially since it lets you see the comparative strengths and weaknesses of each as you level them up.
How long does it typically take to grind up a tank?
In my experience, I've found that it takes pretty much the same amount of time to level a tank as it does to level a DPS, thanks largely to the companion system. As a tank, you'll kill slightly slower than a DPS, but you'll have substantially less downtime, not to mention the ability to solo Heroics while leveling and faster GF queues. A lot of it depends on how much you enjoy playing your tank, since you're going to level up a character you enjoy playing substantially faster than a character you don't.