Ah, I did not realise that this was about a "transformation" between bitcoin and normal money. But in that case wouldn't it make sense to keep the money on the exchange just as long as needed?
The bitcoin exchanges typically require 6 confirmations of a transaction in the block chain before crediting your account with bitcoins. This takes about 1 hour.
If you are speculating on bitcoins and you hear about some news that might cause the price to drop, then you don't want to wait 1 hour before you can sell them.
At the moment even no-fee transactions usually make it onto the block chain as quickly as possible. The ~1 hour (6 block) confirmation time is due to the recipient being cautious that the sender won't try to double-spend the money by rewriting history with lots of computation power. As more time elapses from when the transaction occurred, this becomes harder, and not as a function of your transaction fees.
