Depends on market conditions, and how much you placed the sell order for.

If your sell order is above the current market price then expect it to stay open for a while, because there are no guarantees that bitcoin go up. This has nothing to do with poloniex, since they don't control the actual trades.

If your ultimate aim is to sell your BTC as quickly as possible without any sort of expectation of BTC going up any further, then just place a sell order at the current highest bid price. You'll get slightly worse than market price, but it's instantaneous.