Ask HN: How does billing in man-days work?

Hey guys, I would like to get your opinion on billing in man-days. Take this example:

- you are billing in MD - you have a client who requests some work on Tuesday and you finish it the same day in 2 hours and send it back to the client by the end of the day - on Thursday there's a call from client with a new work, the same scenario happens, you finish it in 2 hours and deliver to the client by the end of the day.

My question is: how much would you charge your client?

1.) 0,5 MD 2.) 1 MD 3.) 2 MD