Hello Nigel. I can't really measure how long it took to get my system right, because it was done in a series of improvements and tweaks (as all good development projects are). In fact, I think that MLO is probably quite perfectionist-unfriendly, because you can spend too much time modifying everything to be just how you want it. Just remember, though, that the important thing is to capture the tasks. If you find a better way of grouping or categorising them so that you just focus on a manageable quantity at any time, you can then add that category (eg flag, context or text tag) or group your tasks into different folders later.
In my case, I found that what worked for me was to minimise the number of different parameters I set for a task.
I use contexts to set Contexts as per David Allen's "Getting Things Done" (GTD).
I use flags just to identify my roles and goals (for my annual review).
My folders and subfolders are organised into four areas of life (personal, home, work, community), then roles (husband, father, friends, home maintenance,,, or electrical engineer, HazOp facilitator, line manager, functional safety engineer), then projects and sub-projects.
I use a fairly limited set of views:
Outline, Task List, projects grouped by project status, tasks grouped by goal, starred or role.
That's pretty-much it. Good luck with setting up your own system - I suggest you only make it as complex as absolutely necessary and spend more of your time doing.
Stéphane