I would not worry so much about the "duty" rating as I would the "head" rating!
A centrifugal pump can run for a long time with no problems. At the heads and flows we are concerned about, there is little problem of overheating from a dead head flow!
The best thing to do is hook it up and then with a hosepipe on the outlet see how high you can raise it and still get a meaningful flow. At some point of height of the outlet, the flow will stop even though the motor is still running. That is called the churn or zero flow pressure. You want to have sufficient flow and pressure to wash the chips away from the tool. I have a couple of homemade centrifugal pumps that I have made that flow on the order of 5-10 gpm (~19 - 38 lpm) and do a great job. The max head of the pump is ~5 ft (1.5 m).