I've been through the full manual for the Mega48/88/168/328 family and see nothing about recommended value for bypass cap on ARef.
I will be switching the ADC on and off (maybe once an hour) so start up time will be important. I could also not find any information about even the order of magnitude of the source impedance for ARef. It always just says "high". But, does that "high" mean 1K, or 10K or 100K or even higher? It would be really useful to know this in order to determine the time constant that results from the capacitor choice. The spec sheet calls out a maximum bandgap startup of 70uS but I would expect that filtering RC to be several orders of magnitude longer.