I think we can get to 100 PH/s eventually just by cutting prices
Assuming:
1) 0.25 kW per TH/s
2) 0.12 USD per kWh
3) 6 blocks per hour
4) no income from fees
then 100000 TH/s requires 500 USD per BTC just to cover the power costs - which is actually more viable than I thought it was going to be.
(across the network, over 26 million USD per year in power costs alone)