I just counted the ppm (pumps per minute 
) (on a B-set, drones on, sounding a first 8ve A) and got an average of 27. I find this most convenient but surely it is a matter of “being used to”. What are your numbers?
Come on now ,be honest, what was the first thing you thought of when you read the title of this topic ?
But seriously 27 per minute for a flat set seems like alot.
RORY
I bet Rory will be the first to post on this one!
But seriously, HJ, it all depends how your bellows gusset is made/how much air your bellows is set up to pump per stroke, and how much/little air your reed takes.
Just on the chanter I can play three-quarters of a jig on one bellows stroke, but with drones and chanter going, and one press on three regs I can empty the bag in about two seconds!
Comes to that that it also depends on your playing-technique, that is whether you empty your bellows completely or play with your elbow fairly far from your body, where you have a much better “lever” for pressure.
I am running elder drones reeds in my B set. Elder reeds seem to drink more air, or least I have not been able to make them be as efficient as cane. for a little bit of extra pumping (the trade-off), I get an amazing tone from elder drone reeds.
It becomes obvious that the number depends on a lot of factors - set-construction; belllows construction; reeds (pressure “hardness” / air consumption) and - as Mike pointed out - “non-set-factors”. I think but that it is not a good idea to construct a huge bellows that only takes (lets say) 10 ppm. You want an “optimum ratio” betwixt volume and pressure (always provided that the bag always is “filled to the brim”, that is). You can´t pump like a fly and neither like an albatros. The thing is a bit comparable to these older bicycles without a gearshift. I think but we might find a “general average” number of a “conveniance compromise” - it might be helpful to bellows makers, pipemakers, reedmakers …
I think that the basic measurement would be: how many strokes of the bellows does it take to fill the bag? That is your starting point. All other factors stem from this. I do not start playing a tune with a full bag; I fill the bag and then start the drones off for a few seconds before playing the melody. THis means I am starting the tune with about 3/4 of a bag full of air, and I then aim to keep that amount in it, until I start regulator accompaniment, when I aim for a full bag at the start of the second time through, and usually aim to have a full bag of air for the last couple of bars of the tune/set to finish with a flourish.
I was curious…so I timed it. I run about 16 ppm. (Concert D set, 3 drones, playing A and drones for 1 minute). I’ve had 5 or 6 different bellows so far and I’ve found the Froment bellows to be one of the most efficient of the ones I’ve owned, although the Whitmer bellows was probably the most efficient and most comfortable (simple but very well designed and made).