You don't have to wait any specific length of time to come back into the UK unless you have already spent 6 months here in the last 12 months.
What counts is the actual amount of time you have spent in the UK in 12 months, not the amount of time you were granted permission for. Each time you enter the UK as a visitor, you are issued with a 6-month visa - even if you are only staying for 2 weeks. When you leave, the 6 months permission to stay is effectively 'cancelled' and then you 'apply' for another 6-month visa next time you come back.
So you could in theory come and go from the UK every month, getting a new '6-month visa' each time and spending 2 weeks here at a time, and you would still have only spent 6 months in 12 months in the UK.
A general rule of thumb though would be to spend at least as much time outside the UK as you did in the UK before coming back - so it shows you are just visiting and not trying to live here... so if you were here for 3 months (Feb-May) and you aren't planning to stay more than another 3 months, you might want to wait 3 months before coming back (August).
If you wanted to stay for longer, i.e. 3 months on the first visit and then 6 months on the second visit, you would need to wait longer before coming back, so that you would not have spent more than 6 months in the UK in total between Feb 2013 and Feb 2014.
