Never heard "block heaters don't last that long". Maybe it's true, but I've had a few, and not one of them have ever died on me.
I plug mine in about 20 - 30 minutes before using the tractor on very cold mornings. It doesn't take more than that to feel warmth coming up thru the block, if it's properly sized and your tractor is stored under shelter out of the wind. I don't need the engine to be 180F to start.
I would not keep any block or pan warmer plugged in 24/7, that's just a total waste in a climate like MA. This ain't Fairbanks, where oil freezes in the pan.