Hi Lizzie,
4-5 hours per night is definitely far too much unless you're in the publishing industry when the quicker you produce something then the quicker you start to make money. Must agree that was taking goodwill way over the limit.
I volunteered to do Boston Stump because I used to live in Boston and I also knew it was a big parish and would take some time to do. But I found it extremely interesting. I found the marriage of John Fisher, one of the founders/father of one of the founders of Fisher Clark, at one time one of the largest employers in the town. I found the marriage of the parents of a great uncle who married into my family. And every time I saw a couple of names I knew what the occupation was going to be - fisherman - because those families were still fishermen when I was young.
Yes, it is possible that the area you're interested in transcribing on FreeREG is complete, but it would be best to email the county co-ordinator to confirm this. I'm not sure if the aim is eventually to be able to double-check the entries. If so, perhaps they would accept you as a proof-reader/double checker for that parish.