In what time increments should web freelancers charge?

I have started picking up some freelance web development, design, and marketing work on the side to supplement my income.

I am a novice at freelance work (although not a novice in the subject matter), so I am only charging $30/hr. I live in Louisville, KY (just to give you an idea of the market in which I work).

My question is: if I spend, say, 30 minutes on something and it’s complete, should I bill for a full hour or break it down? Do most freelancers bill in 15/30/60 minute increments?

I usually bill a total rate before a project but this particular client has asked me to do some follow up marketing pieces.

Thanks for your help!