How long does His mercy last?

Pretty difficult to miss the point here - His mercy endures forever.
Psalms 136:1 O give thanks unto the LORD; for he is good: for his mercy endureth for ever. 2 O give thanks unto the God of gods: for his mercy endureth for ever. 3 O give thanks to the Lord of lords: for his mercy endureth for ever. 4 To him who alone doeth great wonders : for his mercy endureth for ever. 5 To him that by wisdom made the heavens: for his mercy endureth for ever. 6 To him that stretched out the earth above the waters: for his mercy endureth for ever. 7 To him that made great lights: for his mercy endureth for ever: 8 The sun to rule by day: for his mercy endureth for ever: 9 The moon and stars to rule by night: for his mercy endureth for ever. 10 To him that smote Egypt in their firstborn: for his mercy endureth for ever: 11 And brought out Israel from among them: for his mercy endureth for ever: 12 With a strong hand, and with a stretched out arm: for his mercy endureth for ever. 13 To him which divided the Red sea into parts: for his mercy endureth for ever: 14 And made Israel to pass through the midst of it: for his mercy endureth for ever: 15 But overthrew Pharaoh and his host in the Red sea: for his mercy endureth for ever. 16 To him which led his people through the wilderness: for his mercy endureth for ever. 17 To him which smote great kings: for his mercy endureth for ever: 18 And slew famous kings: for his mercy endureth for ever: 19 Sihon king of the Amorites: for his mercy endureth for ever: 20 And Og the king of Bashan: for his mercy endureth for ever: 21 And gave their land for an heritage: for his mercy endureth for ever: 22 Even an heritage unto Israel his servant: for his mercy endureth for ever. 23 Who remembered us in our low estate: for his mercy endureth for ever: 24 And hath redeemed us from our enemies: for his mercy endureth for ever. 25 Who giveth food to all flesh: for his mercy endureth for ever. 26 O give thanks unto the God of heaven: for his mercy endureth for ever.