9 months. Anything from 7 months (early) to 9 months is considered average for crawling on hands and knees. Some infants can take longer and some can skip straight to pulling themselves up and "cruising." (walking while holding on to furniture or walls for support).
6 months. Usually around 6 months is when babies begin to crawl. Remember that scooting, crawling backward and rolling around to get from place to place all qualify as crawling.
8-9 months. Most babies sit at 6 months, and crawl a bit later, at about 8-9 months. Then they stand up around 10-12 months. At each regular check-up, the doctor can follow the baby's progress to be sure progress is within the normal timeframes.
9 months. Usually by 9 months.