It depends upon who makes the rules. There are different rules for, say, unemployment benefit and child benefit.
I'll guess that this question is most likely to refer to Child Benefit and/or Tax Credits. If so, the rule is that
over 12 hours per week counts as 'full-time'. (I've highlighted the word 'over' because there was a similar question about 5 months ago from someone who was studying for 12 hours per week. 'Exactly' 12 hours does not qualify):
http://www.hmrc.gov.uk/manuals/ccmmanual/CCM18 030.htm
The cut-off point for unemployment benefit is, I believe, 16 hours per week. (That figure certainly applies to people who're working, where16 hours or over is regarded as 'full time'. I believe that it also applies to education).
However, individual colleges (and 6th form centres) might apply their own definitions of 'part time' and 'full time'. The cut-off point is often around 21 hours.
Chris