I doubt it will come to look overly rustic, since your inspiration piece is thoroughly cracked and beat up. In fact, I suspect it may not get as rustic as you want it.

Notice a couple of things about the mantle in the picture.

First, the support pieces underneath, which look like they were cut from the same beam that the mantle was cut from, have the center or "pith" of the tree going right through the middle. The interesting cracks on the outsides of the beam happened because of the way wood shrinks around the tree center; if the piece your hubby brought home doesn't include the center of the tree, it won't develop such cracks as it dries.

Second, that beam in the pic was recycled from some other use, probably in a barn, which gave it it's patina. A freshly cut beam installed in a residential setting will never develop that sort of surface.

There could be moisture-related issues if freshly-cut wood were installed against vulnerable wall materials such as drywall. It will take years for a piece that size to fully dry out, during which time it will shrink and warp somewhat unpredictably.

If you want what's in the picture, I'd seek out someone selling the beams from an old timber-frame barn.

Jon, thank you so much for your prompt and informative response. I agree with your last statement about finding an old barn beam someone is just going to ditch anyway. That's really what I had been thinking all along. Then it would already be weathered and narly looking leaving very little work for us to do on it. Kind of hated to rain on the hubby's parade though yesterday. He seemed so proud of that behemoth beam he drug in....not to mention the effort it took he and our nephew to do it. It's a bit on the heavy side.;o)

So I'm still not sure how to proceed. The beam actually does appear to have most of the center of the tree there...it is a bit off center, but mostly there anyway. But you made a good point about it not having the weathered look of something that's been aging for years and I'm not real sure how to recreate that. And even if I did manage to recreate it, would it not be adversely affected by future warping and/or cracking?

Well, I'll just have to ponder this some more.
Thanks again for your advice. I really do appreciate it.:o)

any way to have it dried? Maybe that would speed up the aging process? (it's working for me ;o) )