just about the only thing it'll hurt is your clutch. how long is just one more factor in the calculation of clutch life. I've known people that kill clutches in 20k miles and other cars on the original clutch at 255k miles (honest to god)
if you do it a lot, obviously it'll shorten clutch life. so match revs with the gas/timing and start moving with smooth low rpm to compensate.
1991 Toyota Pickup 22R-E 2.4 I4/5 speed
1990 Toyota Cressida 7M-GE 3.0 I6/5-speed manual
mechanic, carpenter, stagehand, rigger, and know-it-all smartass
"You don't get to judge me for how I fix what you break"
For me, the measure is how "gentle" the bump start is. A gentle bump implies less wear to me. I started the car in 2nd gear the other day, and regretted it. From my experience, bump starting in the top gear is the smoothest. The next top gear is ok, but it takes more finesse and the right speed.
I'm taking the chance, but I can't give you a measure.
The only two things I'm even remotely concerned about due to EOC are my clutch and my ignition switch. A new clutch is only $100 and I can replace it in a day. I figure that bump starting is no more harsh on a clutch than dumping it constantly street racing (my past life). I had fresh stock clutches (and aftermarket clutches) last years while racing - in other words, after putting in a fresh one the first time, I never had to replace it again.
Doing EOC, I use my ignition switch probably about 10x more than I would driving normally (estimating 10 EOC's per drive cycle). I do expect this part to wear out faster, but my factory original on lasted 15 years, so we'll see how long the 2 year old replacement lasts.
I think the concern for wear or breakage for the clutch would be not so much the friction plate material but the springs on the friction plate itself. I don/t mean the pressure plate springs, I'm talking about the ones around the center of the friction disk. I imagine bump starting harsh enough will break them.
I agree that it's all in technique. I smoothly bump start in 2nd doing about 5-10 mph most of the time. I also use a quick out-in, pause, ease out with the clutch pedal to bump start. I choose a higher gear if bump starting at a higher speed.
If you "drop" the clutch, it can break eventually, but this rarely happens.
In my 95 Integra, I dropped the clutch every once in a while just for a 2nd gear shift burnout, and it did in-fact break off part of the spring on the actual OEM clutch. But it also did have 145k miles on it and the car was abused its whole life.
If you slowly let out the clutch, that will be best for it. If you upgraded to a HP clutch (as I did, exedy stg 1) it will take more abuse.
On the never-ending quest for better gas mileage...