About 30% of BEP for most centrifugal pumps, closer to 50% for axial flow pumps. Special consideration for fluids close to vapor pressure, there will be localized heating at the bearing surfaces which will cause flashing and bearing/shaft damage.
Remember all the inefficiency of the pump will go straight into heat, vibration, and noise.
If it's under warranty, simplifies your control system, and you don't mind constant repair/replacement, then sure run it at shutoff constantly.