Open Discussion: How Often Does Google Update its SERPs

This is a question that has been on my mind for some time, and despite researching around the web, I was not able to find a clear and definitive answer.
How often does Google update its SERPs?
That is, how often does it incorporates the information that it gets from its crawling bots and applies it to the SERPs (Search Engine Results Pages)?
I think there are 2 possible cases here. On the first one Google would update the SERPs in real time and constantly, as soon as new information is retrieved from its crawling bots. For example, if a bot was crawling a certain page and discovered that the Title tag was changed, it would immediately report it back to a central data center, that would in turn update the database and the ranking positions for this particular website.
A second possibility is that Google has a fixed schedule to update all the results together. For example, it could do it once every 48 hours, and on every update it would combine all the data retrieved from crawling bots and recalculate all the SERPs simultaneously. The schedule here could be daily, every 48 hours, every week and so on.
This factor is quite important because it gives bloggers and webmasters a time frame to check the results of your optimization efforts. Suppose you discovered that all your h1 and h2 tags were messed up on your site. You learn how they should be properly used, fix the problem, and then start monitoring specific keywords to see if your ranking positions will be affected.