Martin Luther:

When our Lord and Master, Jesus Christ, said “Repent,” He called for the entire life of believers to be one of repentance. [#1 of his 95 Theses]

Sinclair Ferguson:

Biblical repentance…is not merely a sense of regret that leaves us where it found us.  It is a radical reversal that takes us back along the road of our sinful wanderings, creating in us a completely different mindset.  We come to our senses spiritually (Lk. 15:7).…Regret there will be, but the heart of repentance is the lifelong moral and spiritual turnaround of our lives as we submit to the Lord.…

…true repentance can never be reduced to a single act found only at the beginning of the Christian life.  It arises in the context of our union with Jesus Christ; and since its goal is our restoration into the image of Christ, it involves the ongoing practical outworking of our union with Christ in his death and resurrection — what Calvin calls mortification and vivification.  [The Grace of Repentance]