I handle pay per click campaigns for most of my customers. The front-end, setting up and tuning the campaign is the most time consuming part of the effort. Finding the right words and then determining which words best convert. After the initial set-up maintenance requires fewer hours, with much of the time spent, testing new ad copy and adjusting bids to improve acquisition cost. However diligence and attention are important as search phrases should be constantly evaluated and the results fed back into the website to improve landing pages and improve organic results for top performing phrases.
It is interesting to see how people charge for this service. There is an excellent article about PPC management options here