What's the worst-case delay for BTC->USD?
When the BTC funds are on your MtGox account, a market sell order is nearly instantaneous and so done in a few seconds, dependent on the amount you buy.

Depositing Bitcoins to MtGox requires 6 confirmations before they show up in your account. 6 confirmations means 6 blocks to be mined since your transaction, this will averagely take 1 hour to complete.

The time for withdrawing your US dollars will strongly depend on the withdrawal method you want to use.

Note that there are many other exchanges and so this answer only holds for MtGox. It's likely that other exchanges require less confirmations and/or have faster withdrawal methods.