Rear main seal
 

What would cause air to be sucked through the rear main seal? How soon am I going to have to change it?

Huh? Are you leaking oil at the main seal? I don't get what you're asking about air being sucked through.

I have squeal coming from my car. And tge mechanic said that there is a vacuum causing the air to be sucked in through the rear main seal

And he didn't help you beyond that point? Oh man. I don't see your problem as being with the seal itself, but with insufficient crankcase breathing. When the pistons are going up and down, the crankcase develops pressure and vacuum (it pulsates). If you're having constant crankcase vacuum that's making a seal whistle, you have a venting issue.
1) replace your PCV valve
2) make sure your crankcase vent hose isn't clogged
3) find a mechanic who knows his butt from a hole in the ground

Get a new mechainic. He is either pulling your leg or clueless. Even on a new engine the PVC system can not draw enough vacuum to pull past a rear main. If you have ever install a remain you would know that sucking air past it is not easy task and it would require a large amount of vacuum.

Heck just take of the oil cap, does the noise stop? Your engine is vented it will not hold vacuum.

They said the sound goes away when the oil cap is off

See my point #2 above. Actually, read the whole post again. ;)

It's 98 camaro with the 3.8L does it have hoses?

Look for a vent hose coming from the valve cover (passenger side I believe) to the intake plenum.

There is no hose other than the one going to the brake booster. The PCV is in the intake plemun, little black cover on top, it even says pcv valve on it.
http://www.camarozone.com/how-to-rep...valve-in-3800/