There should be a gap of minimum 6 months between the dasha ending dates in the girl and boy's horoscope.
These are general rules and change from region to region in India.
This is a common rule for marriage matching in South India - Kerala and Tamil Nadu - some say 1 year instead of 6 months. This rule is based on the illogical assumption that dasha endings are usually traumatic and this rule will give a decent gap between tarumas. www.reliableastrology.com does not subscribe to this rule as it is illogical.