Jonah 3:10

‘And God saw their works, that they turned from their evil way, and God repented of the evil which he said he would do to them; and he did not do it.'

And when God saw their change in behaviour, in that they turned from their evil ways, He ‘repented' (altered His approach because they were now changed people) of the evil things that He had intended to bring on them, and did not allow them to happen. In other words He showed mercy towards them because of their repentance.

It is, of course, impossible to say how long their repentance lasted. For some perhaps it was permanent. For others it would be comparatively short-lived (compare the parable of the sower). But there was no doubt that for a period at least they had experienced a real change of heart that Jesus said might well stand them in good stead when they appeared before God for judgment (Matthew 12:41). What, however, was more important was that the seed had been sown in their hearts of the need to seek to the God of Israel. It would be up to them how they responded.