For example, for heavily used tables with volumes in the order of 10 million rows that grow by a million rows a month, if the stats are 6-8 months old how detrimental to the performance of the database is this going to be? How often should you be refreshing the stats?
Statistics are kept and used by the query planner, and they have a noticeable impact. I can't give you exact guidelines on how often you should refresh them. That will depend on how much work it takes to refresh them and how much impact fresh stats have on your queries. The real answer for this is to take good measurements and judge options by the results. Tinkering without measurement is a throw of the dice.