A thousand often signals vastness or immeasurable abundance in Scripture — God owns the cattle on a thousand hills, and keeps his covenant to a thousand generations. It stretches the imagination toward God's greatness.
Peter reminds us that a thousand years are as a day to the Lord — his perspective is not bound by our sense of time, and his patience and faithfulness stretch far beyond our reckoning.
The Number Thousand in Scripture
But, beloved, be not ignorant of this one thing, that one day is with the Lord as a thousand years, and a thousand years as one day.