How used is used?
In a grease, both the base oil and thickener are subject to aging. When the soap structure breaks down, base oil can leak out. Aging also degrades a base oil's ability to form a film, and both of these mechanisms can damage bearings.

There are ways, however, to assess the condition of used lubricating greases. Infrared spectroscopy, for example, analyzes grease additives and their ability to resist oxidation and corrosion. Other tests can measure a grease's ability to protect bearing steel (ISO 11007) and copper (DIN 51811/ASTM D130-04) from corrosion, and its compatibility with polymeric materials, all of which vary with age.

Regarding relubrication
Sooner or later, the grease in a bearing will reach the end of its useful life. When this happens, the bearing must be relubricated. Because grease life is statistical in nature, it's impossible to precisely predict when this will occur. Even so, relubrication intervals may be approximated based on application conditions.

Bearing catalogs tend to be conservative in this regard, typically recommending relubrication cycles every six months or so. These intervals are usually based on L1 service life, an empirical measure that indicates the point at which 99% of the bearings tested are still adequately lubricated. Naturally, it's a good idea to relubricate before the existing grease becomes ineffective — either by hand or with lubrication systems.