I find questions like this to be silly. And I don't say that to be insulting.
Who cares how many tanks you have. Hey, if you want to fill your legacy up with 12 Guardian tanks then you go right ahead!
But if you're looking for permission to have more than one tank, let me just throw this at ya: I have multiple tanks, even though I didn't intend to. This is because I found that when doing FP's as a dps so many of the tanks I ran with were so timid and liked to linger at the back. I ended up respeccing my applicable classes to tanks simply so that it wouldn't take three weeks to do a FP cause the tank is hiding in the back.
Regarding how long it takes to level a tank, about as long as it takes to level a dps, or a healer. Unless you're an absolute min/max type of player, does it really matter? How fast you level is going to be more dependent on how many missions you do, not how fast.
Same for me I started with a jugg then I was curious about trooper and pt got one of each and then I thought hmmmm whats left oooh Assasin tank. I love em all but my trooper and bh are my favs