It is based on the last trade.
It could be a fraction of a second or it could be hours (unlikely).
It is rare for there to be a period of time with no trades longer than say a minute.
It could change very very drastically in 30 seconds especially in a panic sell. Generally speaking the intra hour volatility is much lower these days.
Taking a look at a chart might help you visualize the swings and frequency of trades.
http://btccharts.com/#m=mtgox-BTC-USD