The classic rockstar spiky look is the one wherein your spikes have a mind of their own, but not really. this style surely requires a good length of hair say medium to long spikes. If you want the classic look then you should go for longer spikes.
The spikes are arranged in an haphazard manner with the help of straightener and hair gel. This look is classic and its cool. A milder version of the same can be used for maybe college or any casual gathering like day out with friends or a casual date.
The look does require good amount of efforts, time, skill and hair products. Some even get it done by a hairdresser which is the best thing to do as they know how crazy you can go with your hairstyle and can handle your hair better than you yourself can.
This is obviously a young and casual look that will go with your shorts or chinos. It cannot be pulled off by an adult and especially not in a serious environment!