Hayne has changed 3 codes in 2 years. He built his body up to play nfl (a sport that requires almost no aerobatic fitness). He is built like a battering ram at the moment. Probably lost some of his pace, lost a lot of fitness. He is built to make 10-15 carries of the ball over 3hrs.
Your average NRL player will carry the ball 15+ times, while making 20+ tackles, and running back and forth with the defence (10-15m sprints all game).
The skillset and physic needed is completely different (you can probably get a cricket player or swimmer that will offer as much as Hayne at the moment)
He will need to transform his body again. To build up his fitness, his strength and the he will need to relearn skills needed for NRL.
It took sonny 6-8 weeks to adjust to league again. And he was in Union which has similar skill sets. Hayne will take Atleast half a season I would think.
You can't walk away from an elite sport and come back and perform straight away (even Jordan needed time to find his game again).
He was a great player before he left. Was the best fullback in the game and top 2-3 players. Not sure how long it will take him to get back there but I doubt he will get the dally m in 2017.