I hypothesize that difficulty increase will DOUBLE every 40 days from now until the market is flooded with ASICs
If we use this as a calculation for the difficulty rate in September we get:
15,000,000 x 1.20 ^ 4 = 31,104,000 (difficulty on 16/06/2013)
31,104,000 x 1.40 ^ 4 = 119,489,126 (difficulty on 26/07/2013)
119,489,126 x 1.80 ^ 4 = 1,254, 349,053 (difficulty on 1/09/2013)
These calculations presume that:
120 days till September
12 lots of 10
every 10 days difficulty increases
every 40 days that increase doubles
I hope i am wrong but at these levels even a 350Ghz miner in September wont really be a profitable ROI, even if the price of BTC was 350 USD
My view is that you shouldn't base your numbers on difficulty increase solely but also look at what is in the order books of Avalon, BFL etc.
Avalon 150T (500K chips on order) + about 80-100T in batch 1-3 miners (not sure whether chips are included in there twice).
BFL - who knows? say 250T (max would be 500T)
ASICMINER could bring another 200T
KNCMINER - say 150T for first 500 (mix n match of 175 and 350GH/s miners).
Bitfury & Others, who knows? say 500T
We're now at around 100T (easy number) and ≃16M difficulty.
So you'd need another 7400T to reach about 1.2B in difficulty. I don't see that happening right now.