A report by the WSJ on the news that national security adviser H.R. McMaster is set to be fired

Says that Trump has conveyed the message to his chief of staff John Kelly, and an administrative official has said that McMaster's departure could happen "imminently" while another said it could take "weeks, or even months".

The officials cited also say that Trump does not have a replacement in mind, and is unlikely to have a new one before McMaster leaves.

More on the matter here (may be gated).