There are several tests that HMRC apply. I couldn't remember all of them so found this on the web which seems to cover it.
http://www.le.ac.uk/ua/fi/docs/Employment%20st atus%20guide%20and%20form.pdf
As Tom says it is nothing to do with hours.
I would take issue with just one of the items on the list of the weblink I gave you - a self-employed person CAN be paid by the hour or day for services - otherwise it is pretty good list.
Be aware that the risk of being regarded as employed (rather than self-employed) is on the organisation that provides you with the contract for services not on you, the self-employed person. In other words, if the HMRC later decides you should have been the employed, they will seek to claw back employers' NI contribution from the organisation.
Google 'IR35' if you want more information.