It happened to me once, I wasn't in my right shoes, the transaction expired within 8 days
At the absolute worst unconfirmed transaction will expire in about 2 weeks,
probably less 
Thank you for the answer. I've managed to solve out this problem by exporting wallet.dat to another PC and resending same transaction with 0.0001 fee. It was mined in next block, and unconfirmed transaction was overwritten by the new one.