We need to consider the CONTEXT.
3 Above all, you must understand that in the last days scoffers will come, scoffing and following their own evil desires. 4 They will say, ‘Where is this “coming” he promised? Ever since our ancestors died, everything goes on as it has since the beginning of creation.’
Peter was writing around AD 60, as the LAST DAYS of the Old Covenant were coming to an end. Jesus had said in his Olivet prophecy concerning the destruction -
30 ‘Then will appear the sign of the Son of Man in heaven. And then all the peoples of the earth will mourn when they see the Son of Man coming on the clouds of heaven, with power and great glory. .......34 Truly I tell you, this generation will certainly not pass away until all these things have happened.35 Heaven and earth will pass away, but my words will never pass away.
But now 30 years had passed & many of the Jewish leaders who had rejected Jesus had died & the next generation was in charge of Jerusalem. They were boasting that Jesus' prophecy of his coming for the destruction had not happened. They were saying that Jesus' prophecy is proved false. They were living proof of that.
Peter goes on to explain that the time lapse is because God is gracious -
8 But do not forget this one thing, dear friends: with the Lord a day is like a thousand years, and a thousand years are like a day. 9 The Lord is not slow in keeping his promise, as some understand slowness. Instead he is patient with you, not wanting anyone to perish, but everyone to come to repentance.
10 But the day of the Lord will come like a thief. The heavens will disappear with a roar; the elements will be destroyed by fire, and the earth and everything done in it will be laid bare.
Peter is saying that Jesus will come as he prophesied in the lifetime of THIS GENERATION. There is still time to repent, but soon the time for repentance will end. The heaven on earth which Jerusalem typified will be destroyed as prophesied, with all the elements of the Old Covenant. See e.g. Exo. 25:8, Psalm 84, Gal. 4:3,9
Peter continues by looking on to Jesus' final coming for resurrection & judgment & the NH&NE, with the same instruction Jesus gives at the end of his Olivet prophecy.
13 But in keeping with his promise we are looking forward to a new heaven and a new earth, where righteousness dwells.
14 So then, dear friends, since you are looking forward to this, make every effort to be found spotless, blameless and at peace with him. 15 Bear in mind that our Lord’s patience means salvation, just as our dear brother Paul also wrote to you with the wisdom that God gave him.
Mat. 24:35 Heaven and earth will pass away, but my words will never pass away.
36 ‘But about that day or hour no one knows, not even the angels in heaven, nor the Son, but only the Father. ...... 42 ‘Therefore keep watch, because you do not know on what day your Lord will come. 43 But understand this: if the owner of the house had known at what time of night the thief was coming, he would have kept watch and would not have let his house be broken into. 44 So you also must be ready, because the Son of Man will come at an hour when you do not expect him.
That gives the indefinite time - a day as 1,000 years. Our generation still has time for repentance, but the present "millennium" will end, with no further opportunity for repentance.