How many shares need to be mined before a block is released? How many shares need to be processed on average before a block is found?

I know that "luck" can be a huge factor which is why it's not quite as simple as that.

I ask because I'm starting my own pool (www.btcpoolman.com). I only have three people on it at the moment, but we've mined about 100K shares in the past week or so. Assuming I sustain this, how long will it be before I will get a block?

I found http://www.bitcoinx.com/profit/ yesterday, which says that it'll take one year and 138 days to find a block based on 1.2&nbsp;GHash/s (which is roughly what I do when soloing).

Failing that, what should I do if I'm currently mining at the above rate?
When mining in a pool you are simply looking for simpler blocks than those that are used in the network itself. Let's say you are mining in a pool that counts difficulty 1 shares. That means to find share you have to complete 2^32 hash operations. Since difficulty is defined as the ratio between the current target and the maximum (as in easiest) target, each increment in the difficulty doubles the number of hash operations you'd need to find a share. Hence the probability of a share also being a block is:

share_difficulty / block_difficulty


since the current difficulty (29th of May 2013) is 12'153'411.7098 the chances of a share of difficulty 1 being a block is 1/12'153'411.7098, and the expected number of shares until a block is being found is again 12'153'411.7098. Notice that in the last step we are talking about an expected number of shares. As mining is a random process there is no guarantee of finding a block in finite time.