Well, what you want is a temperature switch that opens at around 165, I guess. (depends on your particular furnace parameters). Place that so it senses the water temperature leaving the furnace (not your return water). You may need a lower temperature, it will depend on how accurately you can sense the true water temperature. If I had to do this, I might look for an adjustable switch, like the one on a hot water tank). Wire it in series with the power to the blower. Make sure you put in an override switch to keep it going when you want to restart the fire.
Of course, you really shouldn't run the water below 150-160, they say, due to increased condensation. I'm guessing that if you're not burning anything, there isn't much water vapour around, but I could be wrong.
If I did this, going by my water temperature swings, the blower will run for more than half of that one hour period before it shuts off at 165.
Even though my furnace starts demand at 175, if all my heat loads are sucking at the same time, water temperature gets close to 170 before the fire takes over, so I couldn't cut out the fan much higher than 165. So there's not a lot of savings to be had. But your situation may be different.
There may be good reasons for not doing this, I'll think about it.