Depending on the make and model of tube, somewhere between 1000 and 10000 hours. Ruggedized and mil-spec tubes will last much longer than regular grades, usually.
On this one, the standard 6ZH1P/5654/6J1/6AK5 generally last a minimum of 3000 hours. The mil-spec varients like the 6ZH1P-EV and JAN 5654W are rated something like a minimum of 5000 hours. Sometimes tubes will much longer, but that is a luck of the draw thing.
The 6N6P tubes can expect a minimum of 3000 hours.
Mind you, if the tubes are treated like crap, they'll fail quicker.
With the tubes, it is not a question of if they'll fail, but when. This is why I personally hoard them like a dragon does gold. :)
Price wise, the preamp tubes (6ZH1P, etc) are super cheap. Can find matched pairs for $20-50. The 6N6P cost a bit more, like $30-100 for matched pairs.