Value is limited to the total possible energy expenditure of the human population and our autonamous/semi-autonamous proxies (tools, domesticated animals, etc.). It would seem practically limitless because of the scale of human industry in comparison to the energy access afforded to us by nature in it's current state, but obviously value drops commensurate with potential expenditure-destroying events. Natural disasters, war, etc. are examples, but so are things like drops in consumer confidence and labor strikes.