Despite its name, OpenStreetMap is a collaborative geodatabase (or geodata collection) first, and a map only second (if at all). Anyone with an OpenStreetMap account can edit the data. Live. That is, changes made that way are reflected in the database right away, without having to go though any review or approval process first. Kinda like edits on Wikipedia, but without protected articles.
I was wondering if anyone knows how often OpenStreetMap updates their building footprint data.
Whenever you (or any other OpenStreetMap mapper) edits them.
I'm unsure whether Nominatim works directly on the OpenStreetMap database or whether it (like many other OpenStreetMap-internal secondary services) consumes the minutely diffs of the OpenStreetMap dataset as they become available and processes them to update its indices. In either case, while there might be a slight delay (i.e. lag), there should be almost no perceivable "update cycle". Changes should become visible shortly after having been made in the main OpenStreetMap database.
If your question is how often and when buildings are being edited OpenStreetMap: Well, that depends.
- Most of the changes in OpenStreetMap are made manually, by actual humans (mostly volunteers). They do that whenever they feel like it, for the areas (or even individual buildings) they choose.
- Automated imports (especially mass imports) of external sources are frowned upon in OpenStreetMap for various reasons and thus seldom.
- Manual imports (and for building outlines, you could say that tracing them from provided orthophoto imagery is a manual way of importing) are also done by volunteers and thus mostly whenever they feel like it. (Sometimes "mapathons" are organized, e.g. by HOT, to coordinate some larger-scale efforts.)
- New external sources or new versions of external sources (like orthophotos of an area) may become available (or be made eligible by compatible licensing or permission) at irregular intervals. Each time, that may or may not trigger someone to update OpenStreetMap manually or automatically.
- External sources (like high-resolution orthophoto imagery or cadastral surveying) often only cover specific areas (e.g. one country) not the whole globe. Thus even if everything available was integrated right away, this would still lead to differing update cycles for different regions of the world.