A recent survey found that coffee drinkers drink more than tea drinkers. According to the One poll study, coffee drinkers drink 3.4 cups a day while tea fans drink 2.7 cups during the day. As far as we can tell, there is no data on whether a coffee drinker switches to tea or a tea drinker mixes in cups of coffee.
Here is a press release that says 3 to 4 cups is beneficial.
Three to four cups of coffee a day linked to longer life: Three or 4 cups a day confers greatest benefit, except in pregnancy and for women at risk of fracture.