Its a balance of things. You need to keep actively marketing your site for as long as you have it. You need to monitor what's going on in your site (an hour a day should be enough depending on how big your site is). My monitoring consists of checking for scammers and spammers and any member reports, and those who seriously violate the site rules. You'll find people signing up to use your site to sell products or promote their own business (its up to you if you want to allow that), spammers from Africa and other places trying to con your members, and then a few downright idiots who abuse other members just for fun, plus the favourite of using photos of other people passed off as their own.
You can do all that without getting involved with what genuine members are doing. Its also good to get interactive with your members, as long as what you're telling them is useful and informative, but its not something to be doing every day.