Really depends on the trader.
The media tries to make trading seem this hollywood style industry where we can all get rich if we have a good system. The reality is very different, its a long hard graft and there is no one system that will make you money, the markets "mood" changes everyday and therefore so must your approach, the potential rewards are hollywood but very few traders achieve them.
A VERY good trader could probably average a return £1500 off of 10-12K.
But if you have never traded before, the more money you put in the faster you will go broke.
I would open an account with 1-5K, if after 6 months you are in profit think about stepping your account size up a little.
Dont mean to sound blunt but its safer for your bank balance in the long run if enter trading very very slowly and build up. Some people say you arent a trader untill you've blow a least two accounts, i think thats a little over the top but chances are if this is your first account you will not make a penny for quite some time. Then when you do start making profit you will spend the same time over again getting back to break even. Then on your third attempt you will start building some profits and if you're good the sky is the limit, thats the course nearly every trader will follow.