The city of Madison sweeps bike racks once or twice a year, and the abandoned bikes get sold off in the first week of May. I haven't found out when the University sweeps their racks, but I know they do.

If it's a business's rack, I'd keep half an eye on it. Sometimes, businesses have way way too few racks for the number of people who travel by bike. If the rack would be full even without the squatters, politely let them know about the squatters *and* the scale of the problem. No sane business wants to turn away customers, because that's a great way to go broke.