"The Truth" according to JWs is the religion itself. "How long have you been in the truth?" means, "How long have you been a JW?"
Jesus' words, "I am the Way, and the truth, and the life," are ignored by the Borg. Often, they will link "the truth" with "the good news of the Kingdom" as YKNOT says, but that's just a word game they like to play in the BorgLit. The good news of the kingdom to JWs is that if you want to be a subject of the kingdom (rather than the alternative - DEAD), you need to join the JWs. They believe it is the truth, until you ask them about God committing genocide against little Chinese children and then it's not quite as hard-and-fast "truth" anymore.
I feel like I've posted this Mark Twain quote a dozen times lately. It's a favorite!
"We are always hearing of people who are around SEEKING AFTER TRUTH. I have never seen a (permanent) specimen. I think he had never lived. But I have seen several entirely sincere people who THOUGHT they were (permanent) Seekers after Truth. They sought diligently, persistently, carefully, cautiously, profoundly, with perfect honesty and nicely adjusted judgment--until they believed that without doubt or question they had found the Truth. THAT WAS THE END OF THE SEARCH. The man spent the rest of his life hunting up shingles wherewith to protect his Truth from the weather. If he was seeking after political Truth he found it in one or another of the hundred political gospels which govern men in the earth; if he was seeking after the Only True Religion he found it in one or another of the three thousand that are on the market. In any case, when he found the Truth HE SOUGHT NO FURTHER; but from that day forth, with his soldering-iron in one hand and his bludgeon in the other he tinkered its leaks and reasoned with objectors. There have been innumerable Temporary Seekers of Truth--have you ever heard of a permanent one? In the very nature of man such a person is impossible. However, to drop back to the text-- training: all training is one from or another of OUTSIDE INFLUENCE, and ASSOCIATION is the largest part of it. A man is never anything but what his outside influences have made him. They train him downward or they train him upward--but they TRAIN him; they are at work upon him all the time."