how long will the samsung infuse last?

All depends on how you take care of your phones or electronics in general. I've seen some that just don't care about their electronics. I shake my head at wanna be photographers throw their expensive DSLRs into a large purse or unpadded backpacks But of course, being a techie, we shake our heads at a lot of non technical people abusing their electronics. The Infuse is too new to see how long it will last under normal/heavy usage. The nice thing is you don't have to worry about battery problems (if you have any) and just buy a new replacement. Unlike the iPhone and having to send it in. My Infuse is humming along with no issues for about 2 months now. And that's stock rom but rooted. I'm awaiting for official AT&T gingerbread update so the wonderful XDA dev's have something stable.
 
It comes with a 1 yr hardware warrenty from samsung...so it you get a new phone at close to the end of the first yr and the second phone last a yr..then your 2 yr contract will be up and time to get a new phone.