Renewable lava generation is based in the mechanic of pointed dripstone blocks being able to fill cauldrons with the droplets they drip while having a water or lava source two blocks above the base of the stalactite.¹
Given that a stalactite less than 11 blocks tall has a lava source two blocks above the uppermost block, and a cauldron within 10 blocks under the tip with no non-air blocks in between, with each random tick there is a 15⁄256 (~5.9%) chance for the cauldron to completely fill with lava.
On average, random ticks ocurr every ~68 seconds. With a ~6% chance for the cauldron to be refilled, the average time in between refills comes at around 19 minutes.
In order to build a lava farm, you need to have a contained lava source with a pointed dripstone block set underneath the block under the lava source and a cauldron beneath the pointed dripstone block.
Presumably the blocks used to contain the lava should be non-flammable.
Although it may look like there should be at least one block of air between the pointed dripstone and the cauldron, the farm will work if there is no space between the pointed dripstone and the cauldron.
Note that the farm will not work if there is any block other than air between the dripstone and the cauldron. It will also not work if the lava source is not immediately above the block in which the stalactite is attached or the stalactite itself is more than ten blocks high.