When can you put the rent up?

I have been renting my old house out now for 15mths to a freind of mine the rent is always paid on time but i could do with some extra cash to insure the building and also the interest rate has just gone up how much am i allowed to put the rent up by, she pays �500 per mth for a 3 bed house with garage at the moment so whats the limit and how often can you increase the rent so that we are both happy