Hi Kal,
As I understand there are two primary algo's at work... I like to use the analogy of the prince and the king....The prince goes off and collects real time stuff such as news headlines, trends etc which can appear temporarily on the SERPs - especially on the first page.. So that's multiple times a day when that can happen... Hence this can also explain Google dance i.e. when G tests CTR/bounce rates when considering ranking changes/improvements to a site... in addition to trends/hot news etc..
The king on the other hand then goes through all of the sites, checks authority metrics and other signals and then reshuffles the pack according to G's primary ranking algo for that given keyword... The frequency for this I believe varies across different niches.... A few years ago it was around 2-4 weeks... but these days I'm hearing reports it could be down to a week in some cases... Keep in mind also G's penalty algo's like Penguin/Panda only get executed every so often so that too plays a part i.e. if you have been penalized you wont know if you have recovered until the next round/refresh of penguin/panda which can be months a part...
I hope that helps!