When should part-timers become eligible for benefits?

We have recently hired several part-time employees to gear up for what we hope will be a nice business rebound. Eventually we hope to make them full-timers, but that’s going to take a while. In the meantime, we would like to encourage them to stay by offering a limited benefits package. How many hours should we require them to work before they’re eligible for benefits?—Sari, Dallas

See responses below