What's the life span for a 1990 Nissan Pathfinder 4x4 6 cylinder pickup truck?

It will last as long as you are willing to cover the expense to maintain it and repair or replace parts that break or wear out. At some point, you will decide that the car isn't worth spending money on any more, and THAT is the end of it's useful life.

How long is a piece of string?

Patho will keep going as long as you give it some love. Mechanically these things are monsters. They have three moving parts, none of which ever go wrong and are easy to fix when they do. They are old skool. Get them muddy, hose them out. Unlike modern trucks the only electrics on the things are the lights/indicators, horn, wipers and heater motor. If you're lucky you have a radio. In short there is nothing to go wrong that will leave you stranded in the middle of the desert without IT support to diagnose the ECU, and if it does you can fix it with a rock and some roadkill entrails. BUT.. bear in mind it is an older vehicle. Depending how it's ben used and serviced, it will have wear. If you address it, you'll get much more life out of it. Also, these are small trucks not big cars. Parts are heavy duty and can be expensive. But this is offset by being relatively easy to work on so you save some labour time.