Half inch, to an inch? maybe? I'm not really certain on my size. But it's roughly to the size of my thumbs first knuckle.
Like i said, pumping helps a lot if you can get your hands on the pump.
kinda what I do, is I put my hand over the top hole and push down on it a bit. You gotta figure out your own thing with it tho. Either going up and down with it, circular motions, squeezing the bottom of the sleeve etc. How long have you been on t? I've only been on it for 3 months and it's working good for me.