I teach pretty complex paper mache projects to my third through fifth graders, who attend art 40 minutes per week. I have found that the average project takes about 6 weeks to complete, and have spent as many as 8 weeks, depending on the complexity of the activity. My students enjoy paper mache, and so do I. We have just completed tropical birds (grade 2), fancy hats (grade 3), snakes (grade 4) wild animal faces (grade 5). The problem is basically logistics. I can build and glue paper mache by myself in around a half an hour, bringing it to the point of painting and decorating. That roughly translates into at least 3 class periods, because you waste a lot of time dispensing
materials, demonstrating, helping slower kids get caught up, etc. The remaining class periods are spent painting, decorating, touching up, mounting, or whatever display apparatus you care to employ. Yes, you can do simpler projects, but paper mache tends (for me, anyway) to be a meld of major art concepts, including planning, construction of an armature, gluing, color theory and related materials, so I don't beat myself up on using several weeks to complete the sculptures. Besides, you want a project that is worth keeping, so the kid's mothers can drag them out and embarass them when they are adults!
Hope this helps,