Are the days in Revelation and other prophecies literal?

Revelation is a book of symbol, almost nothing within the vision recorded in the book is literal so we should probably operate on the assumption that time periods in Revelation are symbolic unless we have a good reason for supposing otherwise.

There are prophecies in the Bible where time periods should be taken literally. For example Jonah prophesied that Nineveh would be destroyed in forty days (Jonah 3:4). This was a very clear prophecy – no symbols – so it is clear that the prophecy is literal.