Does the New Testament teach that Jesus’ return was expected imminently (i.e. within the lifetime of that first generation of believers)?
Jesus taught that his disciples should be ready for his return given that he himself did not know the day or hour that it would happen (Mt 24:36).
Other New Testament passages imply that his return could happen very soon (1 Cor 1:7; Phil 4:5; Tit 2:13; James 5:8f).
But other passages teach that certain things would occur before his return, including the destruction of the Temple (Mt 24:2; see also Mt 24:30; Lk 21:11).
In various parable, Jesus indicated that there would be some delay before his return, (Luke 19:11–27; Matt 25:5,19). And according to Jn 21:18 it would not occur before the death, in old age, of Peter. Then again Paul believed that the end would not come until the gospel had been reach all the Gentile nations, Rom 11:12,25.
In 1 Thess 5:1-6, Paul suggests that, although Christ’s return will come as a complete surprise to unbelievers, it should not be totally unexpected to believers.
Perhaps it is best to leave these matters of timing in some tension. There are, after all, good reasons to hope and pray that Jesus would return soon, as his oppressed saints cry out, ‘How long, Lord?’ (Rev 6:10). But there are also good reasons to hope and pray that God will prolong this present gospel day, so that more may seize the opportunity to repent and find salvation (2 Peter 3:9).
See Heiser, I Dare You Not to Bore Me With the Bible, p182.