That depends how many frames per second you're working with. In the UK, we're 25 fps, In the US it is usually 30 fps (29.97).
So, in the UK, 125 frames = 5 seconds. In the US = 4 seconds and 5 frames (5 frames = 1/6th second in the US). So...
4 and a half seconds: UK = 112 frames. US = 135 frames
HOWEVER, changing the default duration of stills in Preferences ONLY alters still that you will import. It does NOT change the duration of stills already imported.
To change the duration of stills already imported, right click on the still - either in the Timeline or Project Panel - and change the "Duration/Speed" setting.
