Indeed, you can keep an enemy marked for the entire encounter if you keep attacking it. That's kind of what you're there to do, to boot! As a Defender, you want to be the most appealing target, and marking (and the attacks you make when your marked target doesn't attack you, or moves) is your main way of doing it.
You wanna watch out for two main things
First, other monsters. While you can mark one dude, most combats are going to involve more than one dude. Unless you can attack everyone each round, you're only gonna have a few enemies marked at any one time. It's up to your party members to deal with the other ones (or to put them within your strike zone with push/pull/slide effects). For this reason, you might want to look into ranged attacks and powers that affect an area (blast or burst powers), since those will help you mark more enemies.
Secondly, monsters that can hit more than one target at once. The dragon might have to go through you to get to your squishy rogue friend, but his breath can get both of you without much of a problem, without a penalty. For this reason, you want to use Monster Knowledge checks to find out what an enemy can do, and you want to position allies at strange angles and positions, so that the dragon's breath can't reach everyone at once. A fireball doesn't care if you're only one of the targets, but if it can't hit much more than you, a monster might not even bother to launch it.
Keeping enemies marked throughout the combat (and punishing them for violating their marks) is your main job as a Defender, and Combat Challenge is what lets you do your main job. So not only CAN you keep an enemy marked indefinitely, whenever possible, you SHOULD!
Happy gaming!