Ideally, angular velocity would be provided in radians per second.
Other than suggesting the use of the metric system, there are not pre-specified values for what is best for a robot arm.
These numbers are highly dependent on the current task and robot. Even your term "heavy" is not specific enough- is it heavy for an industrial arm that can lift up cars? Or is it a heavy gallon of milk?
Lastly, not knowing the robot arm geometry also makes this answer impossible. If the robot arm is extremely long then a small angular velocity would result in a very high linear velocity which might not be desirable for your current task. If there are multiple joints, then you probably want each individual joint moving at different velocities, which is again, dependent on the current task.
If you absolutely have to take a random guess at the best angular velocity to use, I would suggest 0 rad/s.