The NHL has been collecting detailed ice time statistics since the 1997-98 season. One interesting thing that we can see is a shift in individual shift lengths:
This is the distribution of year-long average shift lengths by player, not the distribution of all shift lengths - the average shift length has dropped 7% over the last decade, from a 50-second average to 46.5 seconds. Many of the long-shift outliers - Ray Bourque, Jaromir Jagr, Scott Stevens, Phil Housley, Brian Leetch - are no longer in the league, while Chris Pronger, Scott Niedermayer and Nicklas Lidstrom have shortened their shift lengths. Alex Ovechkin and Ilya Kovlachuk are the rare players who are shifting in the opposite direction, with average shift lengths over 65 seconds the last few years.
It's unfortunate that we have barely one decade worth of data, but I imagine that shift lengths have been dropping for decades - in the 1980s, I believe 60 seconds was the league average. Some of that happened as rosters increased to the modern size of 20, and it has kept going as players have become fitter and talent more equally distributed throughout the league.