"Forty-five seconds is about what the body can handle. A player can go pretty good for 45 seconds. An exceptional player might be able to go for a minute.  At 45 seconds, the player can recover and be ready to go back on the ice at a ratio of one in three shifts. If they go longer than that, it can affect them for the rest of the game."

- Ottawa Senators General Manager Bryan Murray

There's been some excellent stuff floating around on shift length the last little while.  The Bryan Murray quote above is fairly typical of what most NHL coaches say, with the Detroit Red Wings being especially noted for sticking to a set time:

Detroit assistant coach Paul MacLean is never without his stopwatch, clicking it each time the Wings make a line change. "We use our own time," says Babcock, eschewing the arena stat sheet. For playoffs, he wants short shifts -- 40 seconds, tops -- making sure stars like LW Henrik Zetterberg stay fresh enough to sustain the tempo his two-way game demands.

- E.J Hradek, ESPN

The ESPN story linked above is a really good one by the way; it's got some excellent quotes from some of the better coaches in the NHL and looks at a tactical side of the game that's often ignored.  I only saw it thanks to this article over at Jaspers' Rink; an article that pointed out how Alexander Ovechkin takes incredibly long shifts and compared those to other NHL teams.  Most NHL teams don't vary from this much; for example, Oilers forwards under Craig MacTavish had an average shift length of between 39 and 48 seconds for every player; that's actually a tighter range than even the Red Wings (37 - 47 seconds).

Few teams were tighter than the Oilers, but nearly every NHL team fell into a similar, tightly controlled range.  The exceptions were Atlanta, Dallas, and Washington.

In Atlanta, Ilya Kovalchuk led all NHL forwards with an average of 66 seconds per shift, and eight different players had average shift lengths exceeding 48 seconds.  Dallas had ten forwards topping that 48 second mark, while Washington had only two regulars (David Steckel and Boyd Gordon) play less than 48 seconds per shift.

What difference does it make?  I'm not completely sure.  Lions in Winter had a great piece on this that's well worth looking at, focusing on the physiological side of things and making a great point on how this became engrained in hockey tradition:

The 45 second shift rule became popular at a time when it was probably more applicable. A time when players' idea of preparing for the season was drinking 12 beers a day instead of 18 for the month of August.

It's a point worth noting.  It's also worth noting that the three coaches who have made a habit of breaking the 45-second rule (Dave Tippett, John Anderson and Bruce Boudreau) all had long, successful stints in the AHL or IHL, and all have won championships at that level (eight championships between the three of them).  All are exceptional coaches, and I really think they're on to something with this.  It may well be that something currently perceived as a weakness (long shifts) is in fact working in their favour; player conditioning has come a long way and perhaps it's time to have another look at this sort of thinking.