So I know that conventional graphs has us at 7.5 billion difficulty (58000 ph/s) in a year.

However, there is an aspect to consider. It costs real money to increase the network capacity, while the amount of BTC/month that can be earned through the entire network is approximately constant.

So currently, there is: 25 btc x 10 times per hour = 180'000 btc per month up for grab = $54 million (at $300) per month.

So let's see what month-by-month doubling costs. Starting with a basis of 4phs at beginning of November, and let's try and double that each month.

Nov: +4phs = $44m (KNC at $11)
Dec: +8phs = $40m (BitMine / HashFast at $5)
Jan: +16phs = $56m (CoinTerra at $3.50)
Feb: +32phs = $96m (CoinTerra at $3)
Mar: +64phs = $128m (BlackArrow at $2)
...

After this point, there is a very real prohibitive cost to double the network. Getting 50'000 people to buy $100 worth of ASIC Blocks is one thing. Getting 10 million people to spend $100 with no hope of breaking even is something else. The amount of actual money it will take to grow becomes too large for the growth to keep being irrational.

So one of 3 things have to give:

1) Manufacturers will have to start cutting mining hardware prices by 50% per month. So that means a 2TB miner selling a year from now for $25 (1.25c /ghash). I don't see that happening - laws of physics and all that.
2) The price of Bitcoin has to double each month. So $76'800 / BTC 1 year from now. Fun.
3) The network difficulty has to taper off significantly starting after March.

My bet is on #3.