I think it's highly dependent on the firm. In my experience, some of the larger ones will tolerate people at 3-6 months (or more) of "bench" time. I would think it's because of the size and they can't babysit everyone, but automated reporting sure helps catch these instances (and mid-year or annual review time too).

We decided to view it from the opposite direction. 'Bench' time means lab bench, which works well as most of our actual employees[1] are grad students and postdocs. We support their research and have a publication policy that puts their dissertation work first.

When we have interesting RFPs from consulting clients, then we pull people away from the bench.

It's a great way to retain people, have flexibility in the projects we take, and have significant depth and breadth for a small firm.

[1] we also very heavily draw from a pool of highly trusted subcontractors, many of whom are former employees. Because of our experiences with them, we go to them first and they all give us first crack at their availability.