I'd suggest every 120,000 miles. It's the number I've seen around here anyway. It probably will last much longer than that. Mine went to 218,000 before breaking. Don't let yours break. Change the guides as well.
I've seen a couple of posts tonight that suggest changing the tensioner. Are there different types? The automatic spring fed tensioner I have on my M103 motor seems like it should outlast all other parts of the car.
'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)