Not nearly as early as you think and not nearly as early as I'd have liked 

Still, the margin of profitability has always been narrower for new operations than existing ones - which is sort of true in any business.

There are lots of peaks and valleys in profitability in every industry but they do keep chugging along and making money.

I remember speaking with my Aunt a while back (she's an exec for a hospital) about how over the course of her career she's seen new doctors and nurses get hired for grossly different wages and it had little to do with the profitability of the hospital, inflation etc. - the cause was availability. A bunch of people would hear that industry X was "recession-proof" or they'd see quick money to be made and new high school grads go to college to work in industry X. This removes them from the pool of potential doctors so less doctors graduate that year. If this goes on long enough, hospitals start to get desperate for new doctors and their wages increase which of course makes medicine the new "industry X" that everyone goes to college for. There are a handful of industries that "industry X" seems to rotate through but the same basic concepts apply to many things.

In short, when mining becomes less profitable than, say, speculating more newcomers to bitcoin will speculate than mine, which then creates a shortage of miners. Homeostasis, my friend, homeostasis.