Greening, or going green, is the process of buying Victory cards every turn if possible. It usually only occurs in the endgame because otherwise one's deck becomes too diluted with dead cards. However, a player executing a rush strategy may start greening almost right away. In a close game with Provinces running low and both players greening, the Penultimate Province Rule is a good rule of thumb to follow.
When to start greening
There are several factors that determine when a player should start greening. In a Big Money strategy, it is usually prudent to wait until you have enough Gold in your deck before starting to green. How many is "enough" is arbitrary. In a megaturn strategy, one should usually wait until the megaturn is executed. In an engine strategy, it depends on how resilient your engine is to dead cards and how reliable it generates the amount of you want. In all strategies, it depends on what your opponent(s) are doing. Exhibiting deck control and waiting until a reshuffle before greening is usually good play.