Factory allowable clearances, per my factory service manual are;
Intake .006 min. - .015 max.
Exhaust .015 min. - .030 max.
Quote "If measured valve lash falls within these specifications, no adjustment/reset is necessary. Engine operation within these ranges has no adverse effect on performance, emissions, fuel economy, or level of engine noise."
Recommended settings when adjustment is required are;
Intake .010
Exhaust .020
All clearances specify below 140F
Remove injector pump gear cover on front gear housing cover. Rotate engine to TDC, aligning mark on gear with TDC indicator mark @ 12 o'clock.
The barring tool makes this easy, avail. from Cummins for about $40.
In this position, check/adjust intake valves 1-2-4, and exhaust valves 1-3-5.
When finished, rotate engine to place mark on inj. pump gear @ 6 o'clock.
In this position, check/adjust intake valves 3-5-6, and exhaust valves 2-4-6.
Be sure to recheck your adjustment after tightening the lock nut.
I have found no spec. on adjustment interval in the manual.