In Michigan and elsewhere, it is advisable to continue feeding hummingbirds as long as they are present and visiting the feeder. Hummingbirds do not fail to migrate because of the presence of a feeder.
The idea that hummingbird feeders left out too long in colder climates cause hummingbirds to remain behind rather than migrating is a myth. Birds, including hummingbirds, migrate due to an internal clock, not food availability. In fact, individual hummingbirds are on different migration schedules, so leaving a feeder out past peak migration time is a great help to any stragglers. Also, hummingbird feeders merely supplement the birds' diet. Hummingbirds obtain most of their sustenance from flowers and from eating insects.