You and I are limited to time and space, and therefore we tend to think that God must be limited to time and space also. And we think the same way about the devil.
But God is not limited to time or space (and to a lesser degree, neither is the devil). God isn't a material object or a person who takes up space like we do, nor is He limited to only one place in the universe. God is everywhere; as the Bible says, "'Do not I fill heaven and earth?' declares the Lord" (Jeremiah 23:24).
How is this possible? It is possible because God is not a physical object or thing, but a spirit–and as a spirit, He is everywhere. In other words, He is not limited by time or space. Yes, His throne is in heaven–but He also fills the universe.
What does this have to do with Satan? Simply this: The devil is also a spiritual being, and he too is not limited by time or space. Yes, his seat of authority is hell–but as a spirit he can go about "roaming through the earth and going back and forth in it" (Job 1:7).
But never forget this about Satan: He is a defeated foe. Even now his power is not limitless–and some day the final battle will be fought and Christ will be victorious. On whose side are you? Make sure you are on the winning side by giving your life to Christ.