Depends on supply and demand. If green dual bank phasers are a low drop and everyone is making beam boats prices can stay pretty high. I usually price things based on how many of that item appear on the exchange.
I'm goona go with a long long LONG time. There doesn't seem to be any point to buy anything for more than about 15k unless its an extremely rare item and even those aren't really much of an improvement from the common items. And its not worth the time to sell items on the exchange for less than 15k because its a pain to try and see what the average is. The few things Ive tried to sell I put up there for about 2/3 value so I make a profit and someone else gets a deal and I still have only sold about 5 items. Since I don't buy ANYTHING I stopped selling at the exchange since I have no reason as of yet to try and make money.
I can't see a big future for the exchange in just the fact that enemies drop so much loot that anyone can pick up anything from just playing the game. Does everyone think the exchange will the slowly disappear or should they give it a shot in the arm?