|I think the answer to this really varies by user and by child. My first son needed a liner with every OS (and even L Sandys, which are slightly more absorbant) from about 8 months or so until around 17 months or so (this happened to be when he was completely weaned, and then stopped drinking quite as much, I guess). My younger son, on the other hand, has never really needed any extra liners at all; even his night diapers really aren't that wet.
How this can depend by user is how often you change (this varies, but most of use would change every 2-3 hours at that age, I would guess, also depending on predictability of child's wetting/pooping) and how wet you can tolerate. Some people will use lots of extra at night so as not to have to touch wet wings, for instance.