It's hard to say. I've heard of engines lasting 200,000 miles plus and still running like a top and by the same token, I've seen engines blow at 50,000 miles. It really depends on how well they're taken care of.
Are you having any kind of symptoms that are telling you that it's time for a rebuild?
Everything but my engine has gone on me.. well almost everything.. well ok just the tranny and rear diff. My engine still runs like a champ... If i have to rebuild it i would look at stroking it to a 408.. I've heard they can get you better mpg and deffinitly more HP.