if it is kept at regular room temperature (not in the sun though), then it should be good for a month.
I use novorapid (insulin aspart) as my fast acting insulin. Although they say that it is good for 1month, I actually have used 1 vial for up to 2 months, with no obvious change in insulin requirements.
If you notice your insulin is not working so well, then may be worth trying a new vial.
Hope this helps.