It seems their efforts at the end of the war were enough to get them off the hook with Harry Potter wiki saying:
Because he and his family defected from the Death Eaters they were pardoned for their crimes after Voldemort's final defeat and did not serve a stint in Azkaban. He and Narcissa later had a grandson, Scorpius Malfoy, after Draco married Astoria Greengrass.
As Jenaya points out in the comments the Harry Potter wiki may not be the most reliable and so, using their link to Pottermore's page on Draco Malfoy, we can find further support:
Following the battle, his father[Lucius Malfoy] evaded prison by providing evidence against fellow Death Eaters, helping to ensure the capture of many of Lord Voldemort’s followers who had fled into hiding.
A little extra information on their lives afterwards comes from the final paragraph on Draco's life:
Draco married the younger sister of a fellow Slytherin. Astoria Greengrass, who had gone through a similar (though less violent and frightening) conversion from pure-blood ideals to a more tolerant life view, was felt by Narcissa and Lucius to be something of a disappointment as a daughter-in-law. They had had high hopes of a girl whose family featured on the ‘Sacred Twenty-Eight’, but as Astoria refused to raise their grandson Scorpius in the belief that Muggles were scum, family gatherings were often fraught with tension.
So it seems that whilst the Lucius and Narcissa turned against the Death Eaters in the end they didn't become any more tolerant of muggles or half-bloods in the process.