The answer is this: Very Quickly - in some cases less than 24 hours! Naturally, this depends on outdoor temps, wind, insulation, and many other factors, but if you have a catastrophic failure over the weekend, you may not be able to prevent freeze damage by the time you get replacement parts.
Regardless of the age of your boiler. BE PREPARED with a spare blower fan, pump, high limit switch, and solenoid.
Remember, all outdoor boilers are designed to outlast their electronic components. Pumps, fans, solenoids, and high limit switches will all fail during the 20-40 year life of an outdoor boiler (perhaps a few times!).
An investment in the Emergency Preparedness kit will prevent a major disaster for you that could cost you thousands of dollars to fix. Remember, that these replacement parts will be needed eventually anyway, so SAVE YOURSELF from high cost freeze damage!
When you have a catastrophic failure, will you be ready?
Be the HERO to your family and Be Prepared so you can keep your family warm and save the expense of costly repairs.