Shipper,
I've been doing Trimix for almost three years.
1 cc equals 100 units. Therefore .25 cc equals 25 units. My doc started me at 10 units (.10 cc) which proved way too much ... I needed sudafed, walking and a shower ... almost went to the ER. It seems to me that starting you at .25cc (25 units) is very high. I hope if you are starting at this large dose your first injection is in the docs office.
Usually each of the markings on the barrel of the neddle is a unit.
I am not sure what the "average" nor the "maximum" amount used is. I think this is very dependent on your stage of recovery; how sensitive you are to the Trimix chemicals; the Trimix formula you are using; etc..
I use 2 units (0.02 cc) of a 30/1/10 mixture (Papaverine/Phentolamine/Alprostadil). Or 3 units (0.03 cc) of a 30/0.2/10 mixture. For me, the mixture with 0.2 phentolamine does not last as long ... the erection is done when you are, and it feels a bit more natural. The mixture with 1.0 phentolamine will go for two hours ... lasts through a nap!
I'm at the point of prefering Trimix over the pills because of the side effects of the pills (head ache, runny nose, back ache, etc.). Indeed the Trimix is a bit more spontaneous in that you don't have to wait 30 minutes for it to kick in.
Good luck with Trimix and your fight against PCa.