How long do you think you should stay in your first job before moving onto your second one? Is there an optimum time or are there any questions employers might ask if you had been in the first job for a long or short time?

I think you always be in your first job for a year more. After you've been there for a year then just stay as long as you're enjoying it and still learning new skills!

I agree with Feds, I think you should stay in your first grad job for about 1-2 years at least to get that valuable experience. But of course if you like the job don't feel like you have to move on - stay until you feel you've exhausted all your opportunities (you never know, you might find your job for life at a first go!)

It depends on how much you're progessing and gaining from your current position :D As Kite says, if you feel you've exhausted all your opportunities it may be time to move on. You never know, handing in a notice may result in your current employer matching the other offer in terms of salary and responsibilities.

Good luck :D