Good evening Matt...

As far as I know, there is no scheduled rebuild interval for the diesel injection pumps.
The IPs used on your car (and mine) are precision devices that provide many years of service.  

Other than a couple of leaky o-rings, mine has been pretty much trouble free.

There are two things that I can think of that will ensure a long service life.  Those being; 1. Always using clean diesel fuel and 2. Engine oil changes done at the proper service intervals.  The engine oil in your 300D also circulates through your injection pump to lubricate the IP internals, so that's another reason to keep the engine oil changed on a regular basis.  

Some of the injection pump problems I've read about are often brought on by someone who 'fiddles' with the pump while trying to repair a leak.  Although their attempts are made with good intentions, they often remove internal parts and get them out of proper sequence (or lose some of them altogether) and from there on their problems begin to escalate.  

If this happens, or if for some reason you do need to have an IP rebuilt, choose your rebuilder carefully. Not everyone has the know how and tools that are needed to do it properly.    

The injection pump will last beyond the life of the engine unless it is abused with improper fuels (WVO) or run dry of oil.
The vast majority of injection pumps sent in for rebuilding are because of misdiagnosis. The problem is almost always related to injectors, fuel filters, engine compression or dirty fuel.

For those wanting to modify their pump to get more power, here is an aid: http://mercedesforum.com/forum/diesel-performance-49/injection-pump-fuel-adjustment-mw-m-pumps-pyrometer-install-46097/