On average, after babies' bottom two teeth come in, how long until the top two grow in?

1-2 months. The bottom teeth (central incisors) usually come in at 6-8 months. The top teeth come in at 7-9 months. Some babies start teething earlier or later.
This is variable. Teeth eruption is quite variable in normal babies. The average quoted is bottom two by 4-6 mo & 1/mo untill all 20 come in. I have seen the bottom two present at birth or not in untill 15 mo. The top 4 are lined up ready to come down next but again eruption is variable. Occasionally i see outer upper teeth come down before the center 4.
Soon. Usually, within two months the maxillary (top) two front teeth will erupt.
1-2 months. That's just an average. Children do not read the same dento-facial growth & development articles/textbooks that Dentists read. "Standard of Care" is that ALL children have 1st Dental Examination at eruption of 1st baby tooth, but no later than 1st b'day. Recommend Specialist Pedodontist. Also 1st Orthodontic Exam at eruption of 1st permanent tooth, or by 7th b'day. See specialist Orthodontist.