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.