Just some calculations.

Lets say petamine obtain 2% of network hashrate. And there will be 100000 shares.
Lets say at average there are 6.5 blocks mined per hour.
For the next 2.5 years each block rewards 25 BTC.

100% of network mines 6.5 * 25 * 24 = 3900 BTC per day.
or 3900 * 30 = 117000 BTC per month.

2% of network will mine 117000 * 0.02 = 2340 BTC per month.

Finally each share will receive 0.0234 BTC per month.