Short answer: as far as they need. They can assign a rental car for trips no longer than 150 miles or 4 hours. Unless you volunteer for longer, they'll get a taxi, a sedan with driver, or something like that. (If there's a safety issue with a rental car trip, like you not knowing your way through downtown LA, or you're dog tired, you can refuse the rental car.)
From MHT, I can certainly see getting a rental car or driven sedan to any of the Boston-area airports. PWM is under 100 miles, so that's certainly a rental car possibility as well. It's all done during your duty hours, and at company expense (including the fuel for the car). You'll likely take an airline flight out most of the time, though.