If my earnings from bitcoins is more in compare to my monthly salary, then I would quit my job and get into bitcoins as a full timer, it would be fun to work on bitcoin for a full time.
Don't forget the "extra's" you might not be aware of before making that decision. A decent job, at least in the US, also pays your health insurance, short-term/long-term disability, perhaps a pension, or kicks in to 401k, etc., and pays the other half of your social security tax (7.5%). This combined with what is taken out with your Federal and State (if applicable) income taxes, paid holidays, vacation, etc. can easily add up to twice your take-home pay. So say you take home a hypothetical $8,000/mo after all those deductions, you would need to be earning close to $16,000 or about 40 BTC/month to compare.
This isn't limited to just Bitcoin, the general rule of thumb for those wishing to be self-employed and maintaining the same standard of living is to figure on earning at least 2x your net income. For now, I think even if you are earning a decent amount of BTC a month, you would be better off maintaining your job as it will most likely be a more stable and reliable source of income not subject to the wild price swings of BTC.