As is characteristic of numbers in the book of Revelation, the reference is not to exactly ten days (as opposed to nine or eleven). The word “days” as opposed to months or years suggests the period is short, as does also the number “ten” (see Matthew 24:22). Yet that latter number also indicates a full period. The Lord will ensure that the period of tribulation will be overseeable.
10 Do not fear what you are about to suffer. Behold, the devil is about to throw some of you into prison, that you may be tested, and for ten days you will have tribulation. Be faithful unto death, and I will give you the crown of life.