Hi Sandy,
I have been meaning to reply to this for a while now - sorry to take so long!
It probably varies with each child (temperament, health, lifestyle etc) but I never bother to slow the pace for my son. My little guy has just turned 2yrs and he would do LR/LM for hours at a time if I let him! He will get down if he is finished, but often wants to do more during the day. I try to do it twice a day, but really we are more of a once a day (in the morning after breakfast) kind of household. He can learns signs with just one exposure, so I think he is fine with the pace as is. My four year old (typical development), has lost interest in LR/LM,so I am trying to do as much as I can before my two year old follows suit.
Good luck with it!