How large a hole will rain fill with source blocks?

I had a rain storm which lasted all night and for half the next day, no holes were filled with water of any size. I had a small moat I had dug one block deep, but not yet filled with water, as well as a few one block deep holes scattered around. None of them become filled with water.

I don't think it is implemented.

Something that can (partially) fill with water during a rainstorm is a cauldron.

A cauldron has approximately a 15.5% chance of being filled with at least one level of water when exposed to one session of rainfall (Tested with 1000 cauldrons during a regular 15 minute rainfall session).