Go to http://bitcoinwatch.com to find the current hash rate of the entire network. Divide your hashing power by that number and multiply the current revenue per block and 87'600 block/a (the number of blocks per year) with it. What you get is your expected annual revenue if nothing changes.
To do this calculation for you:
Your relative share: 342.56 GH/s / (3118439.45 TH/s) = 1.098498 * 10^-7
For the rest of the calculation, we need the current revenue which is about 14 BTC/block.
1.098498 * 10^-7 * 14 BTC/block * 87'600 block/a = 0.13471979472 BTC/a
However, keep in mind that you only ever mine an integer number of blocks. Because your expected annual revenue is much smaller than the revue of a single block, you expect to mine far less than 1 block per year. This means it's much more a lottery than it is a business if you mine with that 1 miner.