I've never looked at any vehicle as having a useable lifespan. A car will keep going as long as you keep putting parts on it when they become worn or broke. Of course, this eventually leads to the triggers broom scenario.
There is no doubt that driving off road will cause parts to wear out faster but any numbers or percentages put on it would just be completely made up numbers. It depends on different factors, the drivers personal driving style, mechanical sympathy, terrain i.e. mainly flat ground or small dunes or attempting huge hill climbs. The weather is also a factor. If you're off-roading in summer compared to winter, the extra heat will put more stress on rubber parts and oil filled parts such as suspension, engine, gearbox and diffs.
Again, which parts wear out first will also down to the above factors. If you're driving on level ground, the engine, drivetrain, suspension etc will not be taking much abuse. If you're doing a lot of small dunes the suspension and 4x4 system will be working a lot harder, if you're doing hill climbs the engine will be taking more abuse.
Im sorry there isn't a more definitive answer to your question but how long is a piece of string?