As you already know the daily sell limit is 50 BTC. Sell orders are instant in the sense that the sell price is locked at the time you confirm your sell order. As Coinbase indicates on their blog, sell orders are always executed unless there is a problem with your bank account. Coinbase uses ACH to transfer money to your bank account when you sell. Since ACH transactions are not instant, it may take 2-3 business days for the funds to reach your account.