As @Ringandpinion mentioned for me as well its easier to earn money the old fashioned way however in future I intend to use trading as a supplement to income. Considering when to withdraw is a good question and for the moment my approach would be to withdraw at the beginning of the year what you need for the full year. Mind you my conclusion is arrived at on the basis that I intend my capital to be large enough to stretch to the far future despite repeated withdrawals. The income from trading just pushing back that date somewhat or creating some additional buffers for whatever purpose.
The thinking behind it is that it makes for a clean start to the year. Rather than saying I need to make (e.g.) 50K trading, it says I am starting the year with (e.g.) 500K and aiming e.g. 8% a year and hope for a bumper one every now and then and above all avoid catastrophic losses. Then at the end of the year you have whatever you made (or lost) dock 50K from it and start again. This removes the psychological mania - which is powerful in all of us - to trade for trading sake. This impulse is much harder to control than people think - like @Ringandpinion stated it takes some time to get used to trading. Even now I can see that some types of trades I do are more consistently profitable than others and yet I find it hard to disregard the ones which are going south more often.
If you are in addition under pressure of having to make the end of month to meet bills, you will start taking risks that should be avoided. In plain words: trading with money you cannot afford to lose whereas you must NEVER trade with money you cannot afford to lose. In the approach above it is of course not excluded that the principal will be lost but because the money for the year is set aside, you can always go 'back to pa's gas station' and earn money the old fashioned way if you see that come to pass.