Hi i've already posted questions on here about how to obtain a visit visa for my g/f, thanks for the help.

We are planning on asking for a 2 week visa for my g/f, if she is granted the visa but then wants to stay longer than the 2 weeks, will she be allowed to just change the return date of her flight? Or is it not allowed? Would it be a better idea to ask for a longer period of time for her to visit the uk? Thanks for any help.

It wil be a 6 month multi entry she will be issued with, she may stay up to 6 months in the UK.

As TVE says, the visa will (probably) be for 6 months and she would not be breaking any immigration rule nor the terms of her visa were she to stay up to the full 6 months.

However, having originally said on her application that she only intended to stay for 2 weeks she may be asked to explain why she stayed for longer if/when she applies for a subsequent visa.

Technically they cannot stop her staying 6 months, but really perhaps you should have said 1/3 months stay in your covering letter.