> It took me about 6h to develop and iron out the edge cases.
This is where you messed up. Set a hard limit of 1-2 hours, and mention edge cases in your comments and move on. It also depends on the stage of the interview. If you're in the early stages then they probably just want to see that you know what you're doing, and it should be fairly fast with the right frameworks, scaffolding, starters, etc.
Yeah I haven't done this enough to be confident to stop after an hour. Usually there is a lot of experimenting with different designs, adding tests, refactoring, tidying up - I'd think a day for a "1 hour" exercise is normal.
This is where you messed up. Set a hard limit of 1-2 hours, and mention edge cases in your comments and move on. It also depends on the stage of the interview. If you're in the early stages then they probably just want to see that you know what you're doing, and it should be fairly fast with the right frameworks, scaffolding, starters, etc.