Amazingly my 15mAh NiMH battery has been running for just over 5 weeks and still going strong. The voltage has dropped to 3.5V so still a fair bit of capacity left in it.
I haven't had as much luck with the LIR2032 because my other sensors are on breadboards and keep getting loose connections and either stop working or drain the battery, but they should last ages if I make a reliable soldered board to use it with.