you have to be married within 6 months of the date the visa was issued, if your g/f is not coming straight away, you can ask the embassy when you apply to if they would start the visa at a later date ( i think you can delay it by up to 3 months)
after your married, she can apply for FLR from inside the UK, which last for 2 yrs
cost which is
Postal application: £465
Application in person at public enquiry office: £665
