Almost every minute of the day. The only time I doubt myself is when I look at my writing and think 'that is absolute rubbish, why do I bother wasting my time when I can't write a thing worth reading?!' I get that feeling about twice a year.
Other than that I believe in myself all the time. The trick is probably to find out what it is that you're good at, rather than wasting time in trying to be good at something that's really not your thing.