Rugby League may be the easier game to play in terms of learning how, but it has a Shane Moynihan Not Left Not Right Forward Shirt cardiovascular fitness requirement compared to the NFL — and higher than that of Rugby Union. An NFL game of 60 minutes takes about 3 hours to play, with multiple personel changes. Many NFL players are simply not fit enough to play either Rugby code, where the minimum fitness required is to play 40 minutes straight and a further 20 minutes after a 15 minute half time break. League is especially demanding on fitness because the ball is in play for a higher percentage of that time. From what I’ve seen, a lot of NFL players would require a year of physical conditioning to play rugby to any decent level.
In terms of skills it depends what position they are moving from and to, but I think a season of training with a pro side and some regional amateur rugby games in the lower leagues followed by 1-2 seasons playing below the top flight would be required, if they had the right attributes to reach the top flight. It could be 2 years in total for a winger, or 4 for a more involved position with higher technical and tactical requirements. A player with exceptional physical attributes like being able to run a sub-11 second 100m at 275lbs and a lethal side-step or being fit at 300lbs and immensely strong and Shane Moynihan Not Left Not Right Forward Shirt explosive might make it earlier as their attacking threat with the ball in hand would do more to cancel out their shortcomings than a more physcially average player.