I know you can sell your coin when the price is high; but can you trade quickly enough to profit from such temporary surges (buy low then sell high only a dozen hours later) ? What is the average request processing time if you want to buy, say, 10 BTC on mtgox ?
An exchange will allow you to trade instantly; the moment your sell/buy orders go through, the btc/usd is available in your account for usage, however getting USD into the exchanges can take some time and in the case of mtgox, getting USD out is almost impossible (it's why the price at mtgox is about 10% higher than elsewhere). Your location matters most, if you're in Europe you could get money into Bitstamp within a couple of hours if you pay for a fast transfer and once it's in you can trade instantly.
Orders don't take long. Most of the time (i.e. when the exchange is operating normally and isn't down) they pretty much just happen so long as you know, the orders can match with other orders. If you buy 10BTC at market rates and tell mtgox that you don't really care what you pay for it, then it'll just go ahead and do it. There's not much waiting involved other than the web interface refreshing, which isn't strictly necessary since you could trade by API too if you'd prefer that.
buy & sell occurs at an exchange, which is instant as long as the exchange does it instantly. The exchange is keeping tracking of which account inside their exchange has what stuff. So it is just the exchange writing its records to database.
This has nothing to do with how long a bitcoin transaction (sending bitcoin from address a to address b) takes. bitcoin transaction occur in bitcoin network. The buy & sell is just internal to an exchange (eg. mt. gox) and occur outside of bitcoin network.
