24″ is long enough for most mash tuns to check temperature without any concern of getting the thermometer body wet. I have both of these. See: Hands on Review: ThermoWorks 12″ and 24″ Digital Thermometers – RT610B-12 and RT610B-24
As of this posting, the 24″ version is marked down $20 to $29 while the 12″ is marked down $14 to $25. Check each product page for up to the minute price and availability.