Typically, they only fail at fairly high miles on a 7.3. Dealers like to quote the need for a replacement due to high profit for them, but rarely does it fail catastrophically or without warning. Typically, you have some power issues due to lack of fuel pressure long before it strands you. It's far down on my 'worry list'. Dealers also like to replace injectors #3 and #8 due to false indications from scanning software. While they are 'in there', they usually talk you into replacing all 8, especially on high-miles rigs. Don't fall for this one. The FTE forum is littered with people who have had dealers replace many parts before the relatively simple fix was reached. Read up and follow the 7.3 forum.
From what I see in the FTE forum, you're far more likely to be tripped up by the CPS (have a spare in the glovebox), or the IPR, or bad batteries and/or alternator. You should also take various preventative measures like doing the Hutch mod, replacing the fuel bowl drain o-rings, insulating the wiring harness over the pass. side valve cover against chafing, and insulating the power steering hose between it and the driver's side shock tower. Replace the fuel filter every 15-25K miles and drain the bowl monthly. You might also keep the o-ring kit from the 'dieselorings' guy in your glovebox. It's a $65 kit with all fuel/oil related o-rings.
My only on-the-road failure was the starter, for which a Chicago dealer soaked me $375 (part only - I changed it). I had no warning of this, but it stuck in the ring gear (at times) and I had many days of travel yet to follow.
Colo Springs, CO
'78 FJ40, '89 FJ62, '98 4Runner