If your order does not match, this means either:
your desired bid (buy) price is lower than the lowest ask currently on the order book, or
your desired ask (sell) price is higher than the highest bid currently on the book
In these cases, your order will remain pending until it is matched. In other words, it will remain on the order book until an order on the other side gets submitted that would match yours.
A matching order in the case of (1) would be an ask order equal to or less than your desired bid price. A matching order in the case of (2) would be a bid order equal to or greater than your desired ask price.
Pending orders are canceled after 30 days.