A LOT. I'm not sure if I've ever seen numbers, but i've accidentally hooked my 46" dozer under my snowmobile trailer and picked the rear of the tractor off the ground with me on it. I'm amazed at what these units will do.
I'm not sure what kind of weight we're talking when a 42" blower is raised, but it will do it for 10 years, over and over with no problems, given good seals. With good seals, i've never run across something the hydrolift would balk at. They were designed with Johnny buckets in mind, and with the low center of gravity and short angle on the johnny bucket hitch, I bet that you could lift 500#. Not sure what the front of your tractor will take.
I've loaded my loader now with all that the bucket will pick up, and that has to be some serious weight. With me, wheel weights, and my genny off the back, she's feelin pretty light in the tail, but I'm not seeing any issues up front. I don't plan on making it habit, but have wanted to see what it will do.