Normal procedures state that after application of reverse thrust after landing, (decelerating through 60kts) one should wait for N1 values to decrease to 'ground idle' before stowing the reversers. But what is it?
N1 Ground idle in forward thrust (such as idling after engine start) is around 22% N1. After the landing and full reverse is used unto approximately 60 knots, the reverse thrust levers are lowered (but definately not right down) until reaching the idle detent which is about 1.5 inches from full down and which runs the N1 down to reverse ground idle (same N1 22% ). Then when at taxi speed drop the reverse thrust levers to the bottom stops and the N1 doesn't change (or hardly). Except you are now in normal ground idle forward thrust of 22% N1. Approach idle (against the thrust lever rear stop) is around 32% N1 (for heavy rain ingestion reasons).
It stays at that figure (roughly 32% N1) for four seconds after touch down to give you a quicker spool up to full reverse. That is why you should always very promptly select reverse on touch down. If for some reason you either delay or elect not to use any reverse, then after that four seconds the engines reduce from flight (approach) idle (32% N1) to normal ground idle of 22% N1. If you then change your mind after the four seconds and decide to go for full reverse, then spool up times are considerably longer from 22% N1 rather than from 32%N1.
The figures above are for the 737-300 with 20K engines. Note all figures are referenced to N1 not N2.