I dunno, my players usually figured it out after about 3 swings. I've given up worrying about it. I haven't noticed that it really changes anything but to make combat run more quickly by letting the fighter types figure out what they need to roll.
As a general rule, most encounters will have foes that require anywhere from a 6 to a 15 to hit depending on the difficulty of the encounter and how proficient the PC attacking is. So there usually about 10 possible numbers and often, one of the numbers in the middle is the correct AC.
But, players do not figure out enemy AC in 3 swings as a general rule. To figure it out precisely requires that the exact number needed to hit is rolled and the number which will miss by one is rolled. This requires a minimum of 2 swings and the odds of this happening (miss hit, or hit miss) in 2 swings are 1 in 200. This is the only way to determine the exact number without the DM telling a player or giving hints (e.g. "you just barely hit"). The chances of this occurring within 3 swings are less than 1.5%.
So no, your players aren't figuring it out. At best, they are guessing.
Handing out this information only helps with some older style not so well designed systems like THAC0. With a good mechanical system (like 3E BAB where the player merely adds two numbers and tells the DM who does the comparison in a nano-second), the reason to not hand out the number is to not spoon feed the players on the difficulty of the foe.
When a player rolls an 8 and misses without knowing the AC of the foe, he might just figure that he missed, not that the foe is tough. When a player rolls an 8 and the DM tells him the high AC of the foe, then the player suddenly knows "shoot, I need a 15 to hit this guy, the team had better start handing out bonuses right now and we'd better pull out the big guns". Some DMs think that handing out this information is both metagaming and taking the mystery out of the game. Other DMs think that the PC with a single swing would know how skilled his foe is. Personally, I fall into the former camp. I don't think that the PC and hence the player would automatically know how skilled the NPC foe is with a single swing. I think it would take time to figure this out and the best way to do this is to see what happens as the dice are rolled. A 13 is rolled and a miss occurs, then the players can go "oh shoot". Four 6 or less numbers are rolled and the player just thinks that cold dice has occurred and his klutzy PC doesn't really know that he is facing a skilled foe or not.
This level of free information metagaming is not preferable for some DMs. The game is better if there is some mystery in it and the players don't know most everything as if they were playing chess or Monopoly. In fact, 4E (especially as one gets up into Paragon level) is easy enough as is (assuming reasonably well designed for combat PCs and overall party) and doesn't need the DM making it even easier by giving out free extra info.