A lot of us don't ever feed milk ad lib but feed whatever keeps them healthy.
Some preps 1L per day is enough, others it's 1.5L.
Once they can manage 350ml per feed without being so full the milk spills over into the rumen (which is a bad thing), you can put them on 3 x 350ml feeds.
They'll start eating cake when they get hungry and you aren't there with their milk
You are giving them hay, aren't you? They
must have hay in order for the rumen to develop properly.