During Market Hours:
If you place a limit order during market hours and the market price reaches your limit price, the order will be executed on the same day before market close at 3:30 PM.
If the market price does not reach your limit price, the order will be canceled.
After Market Hours:
A limit order placed after market hours will carry over to the next trading day and will be valid until 3:30 PM on that day.
Note: A limit order that is not executed by 3:30 PM on the day it is placed (or on the next trading day if placed after hours) will be automatically canceled at the market close.