I have replaced OEM shocks on various vehicles in the past but always after several years of driving the vehicle. And I was always amazed at how much better the car/truck behaved in every way, especially in rebound situations. So when I got my new F350 in May, 2013, I put Bilstein 5100s on before it was a month old. No sense in waiting. Original shocks would probably do OK for a year or two but the Bils provide so much better control and damping that I decided I didn't want to wait.
I've now been full timing for 2 years, pulling a 14k fiver all over this country and the shocks are doing dandy.
Also keep in mind they have a lifetime warranty.