This time indicator is unclear. Should we read it literally or symbolically? The number of ten days was probably given to tell the original audience that their suffering would be limited (“It’ll only last ten days…”). The number of ten days is normally used in the Bible to refer to a small amount of time (Gen. 24:55; Num. 11:19; Dan. 1:12; Num. 14:22; 1 Sam. 1:8; Job 19:3; Acts 25:6). Moreover, Daniel was tested for ten days, when he was sitting under Babylonian persecution (Dan. 1:12-14).
One speculation is that this was referring to gladiatorial games in Smyrna. Osborne writes, “It is the language of the arena, similar to inscriptions celebrating gladiatorial contests; the games of Smyrna would be especially dangerous times of anti-Christian sentiment, and this could refer to a short period in which the danger of imminent death was great.” However, later Osborne claims that this is too “speculative.”
In conclusion, it isn’t certain why the number of ten days was used. It was probably used to show the limitation of their suffering against the backdrop of eternity. Perhaps we’ll get to heaven, and God will tell us that there was no symbolism at all. He used the number ten, because their suffering lasted that long! However, even if this is symbolic, it probably is simply referring to the limitation of their suffering.
 Osborne, Grant. Revelation. Baker Exegetical Commentary on the New Testament. Grand Rapids, MI: Baker Academic. 2002. 134.