As a Leader or Assistant Leader, the training will consist of 19 modules that can be spread over 3 years. Many of these can be done online or as downloadable workbooks, but some cant. I would plan for at least 4 days of courses during your main training. You will need to schedule time with your training advisor to actually validate the training based on the experience you gain running the colony.
You also need to renew your First Response training every 3 years where at all possible, which is a day in itself.
One thing that is worth considering is events that take place outside of your regular weekly meeting. For example, all Beavers need to be given at least one opportunity to spend a night away each year. There are also district events that you may well be expected to attend (most districts run a yearly sleepover that you can attend instead of running one yourself) and these can number quite a few weekends each year depending on how many your are able to do.
You may also find yourself taking your Beavers out on your own trips on the weekends to give them the best programme you possibly can. In my experience, activities on a regular meeting night are great, but even at 6 years old they crave adventure and need to get out and about.
You might also be expected to attend district meetings, about every three months in my experience.
Admin can take quite a bit of time, and it is not always easy to delegate as much of it needs to be done by someone with a leadership role due to needing access to personal information and finances etc.
It is worth remembering that you will be able to get support with most of this though.
On balance, it is a big commitment, and it will certainly eat into your family time. However, it is so rewarding and enjoyable that you may well be able to find a good balance.