Five to eight years is a good number. Definetively would change complete after the 10 year mark. If you have an old drillstem and starts leaking at any threaded joint, complete failure may be only minutes away.
If considering a hydraulic torquing machine, make sure your cutting deck has enough space available to position the tool above the drum. If you have a top unheading device, like a Delta Valve or Z&J, access might be limited.