It changes day to day. Some days my kids will/would sit for 45 minutes strait happily soaking in everything I showed them, then some days I am lucky to show them a set of ten words in fast flash! If you can engage them and get their attention and you are keeping their attention then keep showing them. If their attention wanders they have had enough. So stop. You will find that your oldest will probably happily learn for longer than your younger kids. Also if they are relaxed ( after bath, before bed) in a quiet mood ( yes they are rare

) then you can usually have more success than when they want to go outside and play.
Overall it would be a struggle to overload a toddler or preschooler but it could happen to a young baby. I can recall a number of occasions when my first was overwhelmed by stimulus at the shopping centers up until about age 1. Remember not to interfere with their regular nap times or eating times. You are more likely to under teach than over teach

Oh and yes they truly can soak it all up! Amazing hey!