Behind the canines are two sets of molars, the first and second molars. These usually are the last to come in. Every child is different, so there is no one-size-fits-all guidelines for the length of time it takes for molars to come through. Instead, a time frame range can help you judge your child's situation.The upper and lower first molars begin between 12 and 17 months. They will be fully in between 27 and 32 months.The upper second molars begin to erupt between 24 and 33 months and will finish between 38 and 48 months. The lower second molars come in between 24 and 36 months and will be set between 34 and 48 months.tart and Duration. Every child is different, so there is no one-size-fits-all guidelines for the length of time it takes for molars to come through. Instead, a time frame range can help you judge your child's situation. The upper and lower first molars begin between 12 and 17 months.