We will be leaving Paris on a Sunday morning from Gare du Nord. What is the metro schedule like between 7 and 9 AM on Sunday? Are there fewer trains? Should we allow extra time for traveling from the rue Cler area? The ratp website was not very helpful.
The theoretical best time of the notionally worst routing is about an hour and fifteen minutes. You'll have either one or two changes, depending on which way you go. Since you've never done this before, you'll have to slow down and read the diagrams at each change and figure out your walking route to the next platform. Allow an hour and a half, an hour and three quarters max, platform to platform.
I think that Ed mistakenly gave you travel time from Rue Cler to CDG rather than to Gare du Nord. Your travel time will be no more than 30 minutes, including walking. Trains run frequently so there's no need to be concerned about gaps in the schedule.
I did, because I can't read. General stupidy may have also been a factor.
Go easy on yourself, Ed! One more thing: If you leave from either Ecole Militaire or La Tour-Maubourg on line 8, you cannot transfer to line 4 at Strasbourg--Saint-Denis because of construction. Ride one more stop to Republique and take line 5 to Gare du Nord. The construction is scheduled to be completed on 3 May.
From the Rue Cler area, take the metro line 8 from Ecole Militaire to Republique; at Republique transfer to metro line 5 to Gare du Nord. Ratp's estimate of the "normal journey time" is 30 minutes.