MY Girlfriend has a two year vistit visa she has been here(in the uk) 5 months has to go back soon because she can only stay 6 months does any one know how long before she can come back?

Visit visas are for visits, not settlement or other long term purposes. A visitor has to show that they are a genuine visitor.

No matter how long the term of a visit visa, most visitors can, as you say, only spend a maximum of 6 months in the UK per visit.

Although not a hard and fast rule, the convention is that visitors should not spend more than 6 months out of any 12 in the UK.

So, to answer your question, although there is nothing in the rules to say that she cannot return immediately, if it became apparent to an Immigration Officer that she may be spending more than 6 months out of 12 in the UK as a visitor then she may be questioned and unless she could produce a convincing argument as to why an exception should be made in her case, she would probably be refused entry.

So, the simple answer to your question is that having spent 6 months in the UK as a visitor I would advise that she wait 6 months before making another visit.