i think if they gave you a rhythm like torsades or vfib, you wouldnt be expected to measure anything since there arent really any identifiable complexes. they might have you measure brady/tachy rhythms... just remember the 6 sec method, and that each little box is 0.04 (one big box is 0.20) and you should be fine.
Eta: this is good website for learning different rhythms--
www.ecglibrary.com, its not too wordy and it just tells you what the important stuff is about each one
The following members say Thank You:
Would you like to comment?
To be honest, I think the NCLEX will focus on pretty basic things. Mine sure did. I think if you get something like that, it would ask you either for your intervention or to identify it's a regular rhythm or not. I can't see the NCLEX getting too specific because there are things that only specilized nurses are able to identify. Mine was pretty basic, and I passed. 
