On the safety side I would say every 5 years but have gone as far as 10 years if there are no cracks and feel flexible. The hardest part of the job is removal/installation of the drive which is very heavy. You need to make sure the alignment is correct as well. I would review the procedure in the manual that can be found under Documents of this site.
1997 2400 SCR
5.7 Vortec / Bravo 2