I use Memoria Press as well.
We start Prima Latina around second grade, at the beginning of the year or midway, really whenever the child is reading well enough to read the simple vocabulary. I help them with the workbook, I don't write for them but I am there to read directions. We spend 5-15 minutes each day, working at the pace of the student.
After Prima Latina is done we move on to Latina Christiana I. Also done at a student's pace. One child has stretched this over two years, and is a great Latin student, possibly even better than my speedy Latin child. I would say the range would be third-fourth grade here. Again, 5-15 minutes per day, or more if you buy enrichment materials.
Next we do the First Form Series. I think this series is excellent and much better than where the Latina Chrisitana books were going. There is ample practice and the text is not cartoony or colorful (I'm all about business). I also allow us to go at the student's pace, however, I've noticed that my students really are on a roll with Latin at this point. They get it. I'd say that it takes 15-25 minutes to complete, maybe more if review is necessary.
I am satisfied with Memoria's materials and will continue. I don't purchase the DVD but every once in a great while I might listen to the PL or LC CDs to hear a word, usually words with "q" in them. I don't think their pronunciation is wonderful but my ears are spoiled by going to mass in a small room with a priest whose Latin was flawless for many years.
I'm considering having my oldest take the National Latin Exam this year or next year.
Personally, I would never consider not teaching Latin. If we are busy, we always finish at least Latin and math every day. It is important for us as Catholics and for the development of the mind.