If you change the default duration to whatever you want and then hold Ctrl while you marquee select the edit points you can then apply the dissolve with the new duration. This works fine if for example you have dissolves on every edit (such as if you are building a slideshow). If you have the odd dissolve here and there in the sequence, then Graeme is right - you can't just change all the dissolves in a sequence to be a new duration.
Delete all cross dissolves,
Set new duration in preferences,
Select clips and hit Apply default transition to Selection.
How to delete all transitions at once.
I find the command + A and Command + shift + A controls to select everything after a point on your timeline to very useful here too on long projects in conjunction with the good advice above from Ann. You have to be a bit organised in your timeline but it helps in many cases. When you get used to these two commands a whole load of stuff becomes easier such as 'scale to frame size' for multiple clips.