If I've not gone off-roading/green-laning/puddle dipping etc and just done roadwork then they're mostly forgot about until a regular service.
If I have been offroading etc then I check them every time and grease as necessary. That also applies if I've been dry off-roading ... dust can make grease into grinding paste and my theory says that if I put clean grease in, then the mucky grease comes out ... :confused2: