The Ammonihahite apostasy was deep enough, and had been in place long enough, that they already qualified for destruction. Amulek declares that it would have happened by now had it not been that there were righteous people praying for them. The implication is that this last opportunity to repent, although by the word of a prophet of God, Alma, was the last chance they would be given.
Therefore, “the time is soon at hand except ye repent.” They are being given their last chance.