It all depends on how you like to get high! Listen to what everyone has said so far... its the trichomes that you want to look at, and it will determine the kind of high (in conjunction w/ strain type) you get. Somebody also brought up plant color (light green to dark green). When it goes to dark green (somewhat of a brown including shriveled up pistils) the life cycle has ended and you should chop regardless of trichome development.
Start taking close up pics of a few bud sites now... a few days later you can compare new pics to those you took earlier. You will know when your plants life cycle is ended or coming to an end by the color change that occurs to the entire plants!