Couple of ways. First open each precomp and press "cntl+K" to access the composition settings and change the duration to match the time of your comp.
Second way would be to select the precomp layers and enanble time remapping("cntl+alt+t), move the last keyframe to the end of the composition and then press "alt + ]" to stretch out the layer. If your precomps are already the right length then going to the end and pressing "alt + t" would do the trick.
Timeremapping though will add frames to you precomp, possibly making effects, video, ect...look book.