Only trade order remains open (and is thus listed on the open orders list) for as long as you have sufficient funds in the wallet and until it gets partially or fully crossed by a counter order.
Exchange order is flagged with tfFillOrKill tag. It is either executed or cancelled immediately.
More information is available here.
This allows the platform to prevent the transactions with a very bad exchange rate to be executed. The rate is real-time and market-based. In a case of bad market liquidity in the time of transaction, the exchange rate can not be properly calculated which could potentially lead to a very costly transaction.