1) I've really done a bit of everything, from low-level market connectivity development to high-level alpha research, management, and everything in between.
2) You can sum up the changes in HFT over the past decade by saying everything and everyone has gotten faster and smarter. Margins are much smaller as as a consequence. There are also many more players - the same piece of pie is being split up many more ways. Many exchanges that were more closed-off years ago (Asia, South America) have all catered to HFT firms now. Markets globally have become more intertwined. Many of these points are secondary to the major underlying drivers of change - the increase in speed and decrease in profit margins.
3) It varies too much by strategy type to really put a number on. A lot of time is spent improving existing strategies as well. In a group you might have 2/3 of the personnel working on existing strategies while 1/3 focus on new strategies. You can roll out a simple trade in a matter of hours, say to capitalize on large global events that create opportunistic trading conditions. Others can take months to research and implement.
4) Again, varies a lot by strategy. Generally speaking, as you increase complexity and decrease latency sensitivity, you increase the half-life of a strategy, at the expense of increased risk (lower sharpe) and longer development time.
. Good questions to ask, that's why I can't answer many of them 
.
