My girlfriend has just moved into my house and brought her cat with her....now my isbt isnt too pleased she tollerates the cat most of the time but makes the occassional lunge to get the moggy...which has resulted in the dog having some nice claw marks on her snout....
Any advice on calming the situation?

I think over time they will sort it out themselves... I have 2 cats and 4 dogs... the cats are the boss, they put all the dogs in their places if they get boisterous

I have 3 cats and 3 dogs living together, the cats are most certainly the bosses....They have put the dogs in their places numerous times...

Im just worried if the cats too slow just once the dogs gona kill it...like no joke Rogue looks like she means business

Hmm not too much to go on there...
Like Ive tried telling the dog off when she goes for the cat/rewarding her when she doesnt...
How long should i give it before admitting defeat and rehoming the cat?

You should always give the cat an escape route if things become to much, you could also try putting your dog on  lead in the house whilst the two are geting to know each other.

Just let them get on with it.  I have had no problems.  Heaps of cats heaps of dogs all survived. Long tall drink of something you really enjoy and enjoy !!