None of the games that I run have / will have post-length rules aside, as StitchWitch mentioned above, from not leaving fellow roleplayers hanging by not providing sufficient actions/speech to respond to in a post, but that's just good sportsmanship.
You mentioned adult-oriented forum sites, so I am assuming that you'd be okay with an adult game?
If so, I am currently running the adult-rated (mentioning it twice so I don't run afoul of the rules) The Halcyon Society (link to the RTJ board -
link to a message in another game ). The basic premise is twofold: OOCly, it's a place for people who write quality roleplay (regardless of length, within reason) to come together and meet other active members of RPOL who also write quality roleplay. Kind of like networking, where it works by referrals into the group that players make.
The other premise is IC that it's a secret society (with actual secrets!) where you can play a guest, employee, or a member (initiate, adept or paragon) and learn about what is going on in the mysterious manor while meeting others who may help or hinder your discoveries.
I've designed the game specifically to cater to people of varying activity levels, but I do ask for 3-4 posts per week as a minimum simply because anything less than that can make it difficult to move a roleplay along in a timely way. Guests are the least plot-necessary, but for those willing to invest time into the game, they'll find themselves rewarded with prestige, access to new areas of the manor, custom portraits, etc.
You're more than welcome to have a look or send a message my way if you have any other queries. We're just starting up so it is a great time to join!