I believe it started out being four times a year, but then after Ennis divorced Alma ironically the meetings with Jack became less frequent. This is mainly due to Ennis now under obligation to pay child support, so he was forced to look for jobs that worked longer, and it was less easy for him to take off time like he used to. When he was married to Alma, remember she took a job at the grocery store because Ennis's meager wages were not paying the bills. She kept telling him to get a permanent job so he could get more pay, but he didn't want to. He refused to find steady employment, because that would mean not being able to take off time whenever he wanted to, so he could go on his trips with Jack.
After the divorce, Ennis had to come up with the alimony and child support, and had no choice but to take jobs he wouldn't have taken. I agree that Jack could have/should have done more to convince Ennis. I know he tried very hard, but I feel there was always a boundary there, Jack was right on the brink of saying everything, but he was always afraid of how the closeted Ennis would react to it. There were moments where you look at Jack's eyes, and it seems he's right about to tell Ennis "I love you, damn it!" Instead, he walks on eggshells the entire time, afraid of Ennis's reaction to his confessions. Ennis was, deep down an extremely vulnerable person. Jack saw this right away, and that was why he was so gentle with him. The big tough guy macho act was a facade, the impenetrable fortress around his heart succumbed to Jack, Ennis was someone who reacted in two ways to protect himself. He either overcompensated, such as he did with Alma and the bikers, had angry outbursts, or he retreated into his shell, quiet, shy Ennis, sitting alone at a table eating all by himself, mumbling when he talked.
Only with Jack would Ennis actually show his real self. Anytime the two protective measures would come out, Jack would quickly break them down. When Ennis was not talking, and all shy, Jack made him feel comfortable, so he talked and confided in him and laughed like with no one else before. When Ennis did the angry tough guy act, Jack wrestled and punched him, quickly showing him that act didn't fly with him. It seems Jack always knew what to do with Ennis, because he understood Ennis like no other person. This is why it should have been easy for Jack to get him to go to Texas to visit at least. I think Jack was the one who set the dates for their meetings, he could have just told Ennis to hop into his truck one day and drive down to Texas or up to Lightning Flat to meet his parents. Jack had wanted Ennis to go to LF with him since that first summer.