Select them all the bin, right click on any one and go to properties and change the duration to whatever length you want. Make sure to hit the enter key after you enter a new duration, then exit the dialog box. That will change all the selected stills to that length.
BTW, if this is a series of stills for an animation, when you import them you can check to import as an animation and then just select the first still. Liquid will convert the stills to an animation file you can put on the timeline with each still being a frame. To use this feature, the still need to be numbered consecutively (eg: pix001....pix100) in their file names.