Coinbase took about 4 days for me. It will vary depending on your bank; in some cases you can do pass-through authentication and be set up in minutes. I have an article here that has a little more info about their process: http://www.bit-buddy.com/buying-bitcoins-on-coinbase.html

BitInstant can be as quick as it takes you to get to a WalMart. More info on there process here: http://www.bit-buddy.com/buying-bitcoins-with-bitinstant.html