Obviously it depends on the airline and their requirements.
My airline requires you to have one years flying experience (not necessarily with them but within the last two years), and I personally believe that should be the minimum of time as it takes most people about six months just to get used to the whole flying-job.
I felt ready to hand my application in straight after the year, but unfortunately I witness many people who just apply because they can, purely for the money, or because they feel it is expected of them.
I urge anybody who considers going for 'Number 1' taking some time to question their own motivation for applying, and to make sure they really know what is going to be expected of them. If they then feel ready, that's the time to apply, no matter how many years it takes.
So I reckon besides the company requirements it depends on your personality and maybe your experiences in general. Just don't let anybody else make the decision for you. When you are ready, you'll know :-)