We explore the high-frequency capability of quantum-well infrared photodetectors using a microwave-rectification technique. We characterize a variety of devices with barrier thicknesses from 234 to 466 Angstrom and number of wells from 4 to 32. Our packaged detectors have a relatively flat frequency response up to about 30 GHz. These experiments indicate that the intrinsic photoconductive lifetime for these devices in the high-biasing field regime is in the range of 5-6 ps.