When should I sell a stock that has taken a large % of my portfolio over time because of the gains?

I've been holding Google for quite some time. It was initially 4% of my portfolio but is now 10% which is starting to bother me. Is there a good rule of thumb as to what to do in this situation? My portfolio at the moment is 70% sp500 index and 30% stocks, with Google taking 1/3 of the 30% allocated to stocks. It seems wrong.

EDIT: misleading title. I'm not intending to sell 100% of my Google shares.