[Leanpub cofounder here.]

We have an unconditional 45-day refund policy. This is so readers of the in-progress book can judge whether they think it has stalled. If so, they can decide whether they've gotten their money's worth, and if not, get a refund in 2 clicks.

In situations where books are abandoned after being dormant for a long time, we can't issue refunds since we literally can't. (We take PayPal and we take credit cards via PayPal Website Payments Pro, and we can only do refunds for 2 months with credit cards.) So, in these cases, it is up to the author to make things right.

This happens very, very rarely though: having actual readers of your in-progress book that you will disappoint if you stop is a great incentive for you to finish.