In A Retrieved Reformation. How can you tell that Jimmy Valentine and any other characters never expected to complete jail time?

Jimmy Valentine was an exception. The other prisoners did not expect to be "sprung" from prison before their sentences were served; but Jimmy had a lot of connections on the outside. The text explains this in part and leaves more to the reader's imagination.

He had expected to stay only about three months, at the longest. When a man with as many friends on the outside as Jimmy Valentine had is received in the “stir” it is hardly worth while to cut his hair.

Jimmy is an exceptional person. For one thing, he makes lots of money in his illegal profession, and no doubt he is generous to politicians and other important people. For another thing, he is exceptionally intelligent, which explains why he is so successful as a criminal. The other men in "stir" are not like him. Jimmy has a charming personality. This also explains why he has so many "friends on the outside." Everybody likes Jimmy. They call him "Jimmy" rather than Jim because they like him. Even the warden likes him.

“Now, Valentine,” said the warden, “you'll go out in the morning. Brace up, and make a man of yourself. You're not a bad fellow at heart. Stop cracking safes, and live straight.”

When Jimmy goes to the place where he keeps a room, the building-owner Mike Dolan greets him cordially.

“Sorry we couldn't make it sooner, Jimmy, me boy,” said Mike. “But we had that protest from Springfield to buck against, and the governor nearly balked. Feeling all right?”

Dolan is one of Jimmy's many "friends on the outside." He represents a whole wide circle of underworld figures, shady politicians, and others who can do such favors as getting a criminal pardoned. 
Jimmy's brains and personality are great assets. But he comes to realize that he can use his personal assets to better advantage by going straight rather than remaining a career criminal. He is getting too well known to many people such as Mike Dolan and Ben Price, the detective. Jimmy decides to move to an entirely different area of operations where he is unknown. There he falls in love and his whole life changes. He is even more successful as an honest businessman than he was as a safecracker.