Belt tensioners last around 100K miles on a W124 (300E series) car. Maybe a little more, maybe a little less. Maybe a lot less if car wasn't maintained well before you bought it.
Parts (if you buy them from PartsShop) run around $325 (I did mine around 1 1/2 years ago) and the labor took me around 3 hours one Saturday.
My mechanic charges around $475 for parts and labor. If you're REALLY comfortable as a mechanic and have a BUNCH of good metric tools, give it a go. If you're not, PAY THE MAN. Also, spend the $30 or so and replace the belt at the same time. IT DOESN'T MATTER WHEN IT WAS LAST REPLACED, DO IT WITH THE TENSIONER.
Scotts Valley, CA.
1993 300 CE Cabriolet (A124) 131K miles
1997 C-280 133K miles
2000 BMW R1100RT 69K miles
1989 300 E 216K miles (sold)