I'm interested in the correct answer here as well and I don't think it has anything to do with Coinbase, but the infrastructure and red tape of the banking system in the US. What happens between the request for a transfer and the actual transfer? I have another online account completely unrelated to bitcoin @
www.prosper.com, if I transfer to or from that account it takes the EXACT same amount of time. I withdraw from prosper, then one minute later deposit to Coinbase. Both transactions hit my bank account about 8 days later a couple minutes apart.
--TJ