As far back as you can manage - back to the launch of your site, if possible. The more responsive you are to each individual user, the more you’ll drive home the human element of your brand.
By and large, people appreciate being acknowledged when they leave a comment or review. Potential users might also scroll back and look at old reviews to help them decide if your site is worth their time. If they see a lot of ignored reviews, it could discourage them from giving your site a chance.