There is no magic number.. it depends on calculations based on your hash rate, and then subjectively your personal acceptance of variance.
Use this site:
http://www.bitcoinx.com/profit/Enter your hashrate, along with other data it will give you an estimated time to mine a solo block. Keep variance in mind, what it tells you is only an estimate. Decide if you are willing to hash away potentially a long time with no reward or do as most people and reduce your variance by joining a pool and receive fairly constant rewards based on your contributions to the pool.
I don't mine BTC anymore, I have opted to mine altcoins for the time being. But using my current SHA hash rate of aprox 38GH, the calculator says on average, I would generate a solo block once every 3 years, 79 days! Due to variance, by chance I could possibly mine one tomorrow, or potentially not a single one for the next 10 years.. It makes much more sense to join a pool, for what I think would be about 0.01 BTC per day at current difficulty, provided I was with a pool which solved blocks frequently enough.
Say I had a huge farm, say like 5TH. Now my expected time between blocks is only 11days 17 hours. Still some time between blocks but if I am lucky I could mine more on average doing solo vs pool so it's not unreasonable to try it.
Final thought, anyone newish to mining or with little hash rate, pool mining really is the only option. Unless you don't mind wasting electricity and not earning anything in exchange for the small chance of mining a full block to yourself. Only good for the gambling type I suppose.
Hope that makes some sense to you.
