Not long...
League is more black and white than Union; the rules are much less open to interpretation, either something is wright or wrong on 90% of rulings, so there's no argument about NH or SH style of reffing (if you don't count dominant tackles). There is almost no circumstances where the ball is contested, not even in scrums (teams usually try to force errors with aggressive defence etc), there are no line outs (if the ball goes out, it just goes back to another scrum), there are 13 players instead of 15 and the position numbers are the other way around (ie: the Fullback is no.1 in RL), a try is worth 4 points, conversion 2, droppy 1 and penelty 2. Tackling is more aggressive and dangerous due to the 10 metre gap and legality of shoulder charging, you can't ever kick out on the full, not even in your 10 metre zone (thats something that may take a bit of getting used to, you always run the ball in league, even if when your deep in your own half) and you get 6 chances with the ball before it's turned over (as a result teams usually kick before they get forced to turn it over).
There are more rules that you'll learn along the way, but those are the main differences.
