All these answers are right and wrong. The depths you are hearing about are for their ground conditions not yours. Find someone in your area that has an AT. Ask him or her.
Your answer will be more accurate. In my ground, Southern California and Arizona most of my targets are less than 4 inches. That is probably pretty common for most every where if the truth be told. Now relic hunting is probably a little different, but not much. I'm a coin and jewelry hunter. Relics are usually bigger targets with more mass. They may read better than coins at greater depths. I just hunted all the way from California to Illinois and back again. I found less that 4" to hold true concerning depth. Once in a while something will be a tad deeper, but very rarely.
I have one of most all the high end detectors. Whites DFX, MXT, Minelab Excalibur 1000, CTX 3030, Fisher gold bug II and the AT Pro. The AT Pro is the best detector for the money for what I do, in the detector marketplace today. It's not a CTX 3030, it's not an MXT. It is the most powerful, easy to use, easy to learn metal detector for the money spnt.
Need I say more?
Buy one from Bart at Big Boys Hobbies. Great service, great price, guaranteed.
Garrett service is on a par with Whites if you ever need it.
Thanks for reading--------------------------Good Luck-------------------Happy Hunting