You can get the latest metadata from the org using Illuminated Cloud's integrated metadata retrieval feature. You can access that feature using the menu bar (
Build>Illuminated Cloud>Retrieve Metadata...), context menu (
Illuminated Cloud>Retrieve Metadata...), or keyboard (
CTRL+ALT+SHIFT+Y on Windows/Linux and
CTRL+OPT+SHIFT+Y on Mac). Once metadata retrieval is requested, you can choose the scope of the retrieval--project, module + dependencies, module, selection, or custom. To retrieve metadata not yet in the local project, you must currently use one of the first three options. To refresh a smaller, specific subset of the metadata, you can use selection (inferred from the currently selected file(s) in IntelliJ IDEA) or custom.
As for how often you might do this, there's no single answer. I generally develop in a dedicated dev org, so I really only do it when I've made changes in the org using the Setup menu, e.g., creation of custom objects and fields. For those collaborating in the same org, e.g., a shared sandbox, you might want to do this more often. There are generally two main reasons you'd want to retrieve others' work: 1) to resolve/merge changes in the same metadata; 2) to get access to changes upon which yours depends. For the former, Illuminated Cloud features conflict detection on deployment by looking for modifications to metadata requested for deployment by other users in the org. As long as each user has a distinct login (I have an enhancement request to handle this for shared logins as well), you should be able to deploy whenever you want and only retrieve when you need to resolve/merge. For the latter, team communication is going to be your best cue. You might also consider using a version control system as your conduit of exchange rather than the org.
Hopefully this answers your questions, but let me know if it doesn't or if it just raises others!