Confirmation time of received transactions is a key service level measurement.
For someone processing transactions with their own custom client, it's mostly up to them as to when they consider a transaction confirmed, as there's a trade off between speed and risk.
Multi-node listening allows a reasonable level of confidence that a double spend hasn't been performed, but it doesn't protect against a Finney attack.
Because a Finney attack is difficult and risky to perform (the attacker risks losing the reward for a solved block), it would only be worth bothering with a particularly large transaction. This means the level of service differs by transaction.
A table indicating confirmation time (and thus service level) would go something like this (figures are educated guesses; perhaps someone can measure it an update the answer at some point):
Transaction size Risk of Finney attack Confirmation technique Confirmation time
Small Very Low Multi-node listening <3 Seconds
Large High 2+ Block confirmations 20+ minutes, on average
Very large Very High Many (x) block confirmations x*10 minutes
Obviously it's open to interpretation as to what is "small" and "large". As previously stated, the trade off between speed and risk will help determine a suitable balance for each scenario.