The automatic time step was extremely low, something on the order of 1e-5 seconds. I chose a manual time step (in the Calculation Control Options) of 1 second and got a reasonable progression (I didn't run until convergence, but I don't think there would be any issue.) This is what the goal plot looks like:
Looks to me like it will take a few hours at this rate to reach -40C.
One thing I noticed, you don't need to use a fluid subdomain here. Just set your working fluid as the default fluid.