DONE DO IT !!!!!
If someone you don't know (a stranger). Asked you for a very difficult favor ( which may take months of your time ). What would be a good reason to make you agree and do him the favor ( no money as an exchange includes )?
As a first approximation, I think it is totally unreasonable and I cannot imagine a circumstance under which I would undertake such a task.
I guess it would depend on who is going to benefit from the favor, and how difficult it would be. My neighbor recently gave up a lot of family time to be able to help build a playground two cities over. There were nights he didn't get home until around 11 pm. He was up and driving his daughter to school and working his full-time job, before he was back at the playground. He gave up weekends as well. Plus it was physical work. It took a better part of a month. This left his wife to deal with two preschoolers and a young teenager on her own.
The playground is one that all children can use. Children with disabilities can play along side with children without disabilities. It made their life and bit tough for a month, but the payoff of helping others is big. I guess they figured they are thankful to have normal, healthy kids, and this is a way to give back.