With the current volume of unconfirmed transactions and the increasing transaction fees it is sometimes difficult to determine the required transaction fee. If you use the default value of your wallet your bitcoins may be locked up in an unconfirmed transaction for several weeks. In order to determine the required transaction fee you need to know the transaction fees that are currently required by the miners. I found a nice site that gives a very clear picture of the transaction fees that are currently mined and the transaction fees that are pending in the bitcoin mempool: https://dedi.jochen-hoenicke.de/queue/#24h
The picture for the current week is as follows: