Approx 2800 to 3000 at idle. At WOT, it depends on the motor. A decent trimmer should spin at around 12,000 rpm.

 

Is that the head or engine? I thought the gear boxes were slower than the engine.

 

More like 6~8,000 rpms.

GOOGLE some owners manuals from diff. units, it should be in the specs.

 

Here is a link to a Honda trimmer specs. It looks like the gear ratio is 1.235
Based on that, whatever rpm you pick as a baseline for the engine, you can figure out the head rpms
http://powerequipment.honda.com/trimmers/models/hht35sltat

 

That would be around 8000rpms, my 4500rpm motor mist likely won't work. I'm trying to set up an electric trimmer mounted to my mower deck.

 

Here is a cordless trimmer with 7000/5000 head speed
http://coreoutdoorpower.com/?post_type=portfolio&p=1821

 

That answers the question even better, I've run one of those and they aren't very impressive, too slow. The motor I found was strong enough and 12 volt but only 4500rpm. I would like to be around 8000rpm and attach a fix head to it. Then make a liftable mount on the front left side of the deck.