We are also doing Level B with my 7yos and I’m constantly amazed at his grasp of numbers and ability to do mental addtion, not because he is a whizz at math, but most certainly because of RightStart and the way they teach.
I have not found there to be lots of prep work for Level B, at the beginning of starting this level, I got all the appendixes copied and filed away for when we need them. I have all the cards/manipulatives in a basket ready to go. I will try and just skim over the next lesson before we start or the night before just so I know what’s coming.
Depending on the lesson, we can spend anywhere from 20mins to an hour on a lesson. If it’s the introduction of a new concept, we’ll take it slow, keeping the lesson short. Some lessons have fun activities that are really very easy, eg. playing with the geoboards, drawing shapes, or playing card games. So, in these lessons we’ll just continue until we are finished or until I can get my son to stop!
However, if I feel he hasn’t mastered a new concept yet, we’ll ‘camp out’ on that lesson for a week, doing extra practice on the abacus, playing extra games from the Games book (another great tool if you decide not to go ahead with RS) until I’m happy for us to move on.
We do math 5 days a week but most RS moms that post on the RS group try and teach the child, not the level, so Level B & C for eg. may take longer (and usually do) than a year. There was another discussion about this topic just recently on this forum, if you search for RS, you’ll find hundreds of conversations! Sorry, I don’t know how to get back to that conversation & get you the link without losing this post 
I haven’t used MUS but did look into it extensively before we found RS. To be honest I’m not sure if you will get the complete benefit of the abacus when using another program that does not teach the strategies that RS does. Not saying it can’t be done but just not sure, although supplementing with the games book may help alot as they do include the abacus too.
I like what Sonya always asks: What is your goal for your child in learning Math? My goal is to for him to really understand Math, and enjoy it, not just knowing it by rote memorisation, but to grasp it at a deeper level. I also love teaching it to him, in fact it doesn’t feel like I’m teaching at all as the lessons are fully scripted. It’s so exciting to see the light bulbs go off in his head and his delight when he answers a double digit equation in his head. I think he is a math/science child, which is where his interests lie too, so I want to be sure he will be prepared to do higher math courses for university should he so choose.
The other option is to try Level A with your 5 year old, the early lessons will probably be easy for him but you will be able to see how it works. There is a RS yahoo group RightStart@yahoogroups.com that you can also join for more information if you wish and ask about used materials as they are often being sold on the group.