That's a tough one to answer because you need to be able to figure out what sort of percentage gains you are making month on month. You can only do this by trading for several months (if day trading) or several years if you are more long-term. E.g. If you can make 5% a month (compounded), then in a year £40K will bring in just over £31K which you should be able to live off.
My advice would be to not even think about turning a profit and going full time. Start with a small amount of capital, risk no more than 1% on every trade and analyse every trade you make in detail. The average amount of time it takes for people to become profitable is between 3 and 5 years. So, don't worry yourself with going full time just yet.