Your limit order will get executed on the same day as placed and is valid till the market closes (3:30 PM). If you place a limit order after market hours, it will get executed on the next trading day and will remain valid till 03:30 PM of the next trading day. 

Your limit order may or may not get executed on a particular day. If it does not get executed on the same day as it is placed, it will get canceled when the market closes on that day.