Can you tell me what mileage I should expect to get out of a '94 Ford Taurus with a 3.8-liter engine before having to replace the timing chain?
That's a tough question. The timing chain, unlike a timing belt, is not considered a maintenance item. Therefore there is no recommended interval to replace it, and technically it should last the normal life expectancy of the entire engine.
The life of an engine is determined by the quality of engineering by the manufacturer along with driving conditions, climate, and maintenance.
Timing belts are replaced as part of a normal maintenance schedule due to the frailty of the rubber compounds, as opposed to the steel timing chain and sprockets. In other words, don't worry about it right now. If it's not broke, don't fix it.