Ask HN: Where do you draw the line when interviewing?

I'm interviewing at a startup and they asked me to do a task as a test of my abilities.

I didn't mind at first but I've spent a lot of time the past few days researching and working on it and following up with them.

If they were paying me for my time it wouldn't bother me as much, but they've basically got me working for free - and now I'm not sure I even want the job.

Where would you draw the line with this sort of thing?