how long does a v6 engine last?

How long is a piece of string?

Depending on use and maintenance many years.

It purely depends on how well it is maintained, in the sense of regular servicing and not abusing it.

Any good make of engine manufactured in the last ten years or so is quite likely to last 250,000 miles or more, _if_ it's looked after properly.

Examples of abuse:
Threshing it before it's properly warmed up - it typically takes 15-20 mins or so for the whole engine to get to an even operating temperature, well after the temperature gauge shows it as normal range .

Using full throttle at very low revs.

Excess revving - holding it at the "red line" for more than brief periods, even when hot.

Reasons:
Using a lot of power while the engine is warming up produces a lot of localised heating, which can cause excess stresses and warp parts of the engine. It's a common reason for head gasket failure in some designs of engine.

Both high throttle at low revs and running at very high revs, put high loads on the crankshaft bearings and the oil film that protects them, causing faster wear than normal.

That is down to how it is built and cared for.
Miles do not kill engines but bad maintenance and abuse does.
Do not abuse it. Service it with quality oil of the correct spec on schedule. Not only viscosity but also all the other numbers matter.
Use an OEM quality filter.
If not abused it can easily run over 500000 miles.
Paying for great care saves money later.

Pretty much any engine should last 100,000 miles from new and can make it to 200,000 miles without any major problems, if you take reasonable care of it.

With proper scheduled maintenance, and reasonable driving, it will last longer then the driver. Without proper maintenance and abusive driving, it won't last a year.

It varies.

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It will last as long as the owner continues to put an effort into caring for the vehicle and until the engine is old in terms of mileage. If the owner is careless, the engine could last only 6 months by never refilling the oil. If the owner was responsible, the engine will slowly fade away after 100,000 miles.