That the duration of the dragon’s imprisonment is marked by “years” (instead of “days”) points to the lengthy duration of his sentence. The number “thousand” is not meant to be literal (as in: 999 + 1, no more than the words “chain” or “dragon” or “key” are literal) but symbolizes completion, perfection. 1000 is 10 x 10 x 10 (10 to the power of 3). So, for a complete (or perfect) period of time Satan is a chained prisoner, not free to do as he would.
2 And he seized the dragon, that ancient serpent, who is the devil and Satan, and bound him for a thousand years,