Most will tell you it's 50+ years. The bigger problem is poorly engineered system. Your well field will likely increase in temperature as the field ages due the heat you're putting in to the ground. You need to design the well field for the ground temp in 50 years, not today. Do this, and it should last forever. Especially if you do borings with grout.Interesting. I was thinking more of a system to augment heating and cooling, not a stand-alone. I wonder what the life expectancy of those parts is. Would suck to drop $20k on geothermal with the idea of a 5-7 year payoff, then find that you have to replace the loop in 4-5 years.
edit: I would also expect the installation to be somewhat more cost effective to do when building rather than as an upgrade.
Indoor equipment is typical HVAC life expectancy. 15-20 years.
Also, it's much cheaper to do a horizontal coil loop. You'll need more loop, but it's much cheaper than wells. If you're cleaning/excavating a lot, install this when the earth moving equipment is on site.