A lot of numbers get thrown around, in determining "life expectancy", you may need to be more clear on what you actually mean. A lot of standby packages can last up to 20 years as long as properly maintained, after that time, while the prime mover (engine) can still continue to provide the expected power and response, auxiliary equipment, especially cooling systems, will likely require some form of either major maintenance or replacement. Also, depending on environmental conditions, such as high humidity, coastal conditions, extreme weather, things like radiators and exhaust systems may suffer and require earlier attention.
Also, the impacts of air quality regulations will have an increasing impact on determining how long a unit can remain in service. You don't state where you are considering locating the unit, but in areas with strict environmental regulations, units may require replacement if the have a large amount of repair work to be done, or no longer meet an exhaust emission criteria. Also site factors can force replacement of a unit, such as an expansion requiring additional capacity, and to secure permitting the existing unit must also be replaced.
How you operate and test the standby unit will also have a huge impact on its actual service life, excessive run times at low loads will cause poor combustion, reduced response, internal deposits and slobber. Most standby units are required to start, quickly ramp to rated speed and assume load within sort times, typically 10 seconds or less. Environmental conditions play into this as well, units that require weekly test runs and spend a lot of their time at low ambient temperatures typically show an increased number of failures due to slow supply of lube oil to critical areas, like the crank bearings and turbo's. So additional equipment like lube oil heaters may get added, but these require higher operating cost and regular maintenance as well.
Maintenance for standby units gets deferred quite often, units typically get used much less than a 100 hrs/year, so owners questions need for annual oil and filter changes, regular replacement of belts and hoses, and how often starting batteries actually get changed. It also quite common when looking at a failed unit to find it never had an initial valve adjustment, or other recommended early life checks that are suggested, especially on newer high speed high output lower emission engines.
Generator ends are often neglected, am seeing more and more units with the stator space heaters either turned off or not installed at all. And they are rarely properly tested on a regular basis, so pretty much as long as it starts, makes rated voltage and frequency, and will carry a small portion of its rated load, its called "good". Like engines, generators rated at a certain size used to be much larger and heavier, and way more robust, especially on standby rated units. Todays gen ends tend to be smaller, lighter, with less robust insulation packages and smaller bearings, unfortunately they are mostly neglected until a failure occurs.
So for your question as stated, best answer is "it depends", maybe providing more specific info, such as type of unit, where you are located, environmental conditions, how packaged, how well supported by local dealer/distributor and installation specifics, a better answer can be had.
Hope that helps, MikeL.
