The electric fuel pump on an 89 Chevy truck should only run for a few seconds with the fuel line disconnected. The pump is designed to build pressure, and if it doesn't detect sufficient pressure (because the line is disconnected), it will shut off to protect itself from damage.
More than 15-20 seconds is excessive and indicates a potential problem.
If it runs much longer than that, you have a faulty pressure switch or another electrical issue preventing the pump from shutting off properly. Continuing to run it could burn out the pump.
