People see an email 6 months old and just delete it, because it's "too old" or they refuse to reply because "the customer will think we're a joke taking this long to do something!" This is a great way to lose business, no matter how long it takes to do some things, it's always better to do it than not.
Sometimes people send an enhancement suggestion for a particular product, but their requests are put on the back burner until other important issues are dealt with. It might take 12 months to implement that change, but when it's done, you will email the customer and send them the link to the new version. It's highly likely that their need still exists, and they'll realize that their ideas and suggestions are not ignored.
Reply to customers regardless of how long it takes to respond. It shows you value their feedback, and it's highly likely they'll give you some more.