In the aerospace world, we would run a qual-test 600 sec per axis. This usually is adequate unless the failure mode can't be excited in a single axis. Most large shaker tables can be turned over on their sides to excite the x or y direction as well as vertical z-dir. But, there are such things as multi-axis shaker tables which can excite in more than one direction at a time. But, usually the equipment is mounted at an angle (e.g. 45 deg assumes equal loading in two directions) to the direction of excitation. This gives a component of vibration in two axis of vibration using a single axis of excitation. Add another angle to the fixture and you have tri-axis excitation.