Go to menu: Composition>Edit Comp & edit the end time/frame. Hit OK
To add time to a composition:
1) Make sure that the composition you want to alter is the active composition in the Timeline
2) Choose Composition>Composition Settings..., or press Cmd/Ctrl-K
3) Look in the bottom of the Composition Settings dialogue box for the section labeled "Duration"
4) Change the timecode in that section to the length that you wish the comp to be.
Alternatively, if your comp is already of sufficient duration, it may be that your time ruler is zoomed in too far. In that case, press the +/- keys on the keyboard (not the number pad) to zoom out of the time ruler.
Hopefully this helps!