When is the metadata store updated?
Initially, the indexers have to process the entire disk so there are hours (days) to take one pass to index all files. 

Steady, state the kernel and system indexes all files as they are written so newly created files and files copied in are basically indexed real time. 

You can see this in action by having spotlight in finder search a nonsense string that doesn’t exist. Then open text edit and past that string into a file and save it to your desktop - watching the finder window update to show that file.