Limit orders execute at a set price or better, dependent on whether you buy or sell.
If you’re buying, they will be executed at the set price or lower
If you’re selling, they will be executed at the set price or higher
This means they are not guaranteed to execute and must be placed on whole shares.
A limit order can be placed at any time, but if it is not executed by the end of the US trading day, it will expire.
Finally, limit orders will be rejected if they are more than 5% above (for buys) or 5% below (for sells) the current market price.