I haven't really thought about changing it but no more winching than most people do, it should last a long, long time.
This is just a educated guess but I'd say you could probably pour some diesel in there, run it a little to mix it up, and the diesel would dilute it enough where it would run out the drain.
I don't recall what I paid for the last I bought (it's been awhile) but I can say from experience that almost anywhere is going to be cheaper than a JD dealer (except maybe a Cat dealer). Just about all the lubricant brands have the "corn head" grease available, they just call it something else. I think "flowable" grease is the generic term used. Now JD does claim that their grease is the best, and it may be, but you have to consider the source of the information.