my experiences tells me its really hard to get people to play playoffs (depends on how u can motivate them) so 2 months is best because u get less playoffs to manage and it makes reaching the playoffs more important.
id also suggest no playoffs for free beacuse its jsut too many, id also considering only displaying free standings with the schemes that arnt classic, so then you can see the top players of 'none league schemes' and maybe rank the top 3 players in a hall of fame.
But with classic, don't make too many playoffs because its hard to get people to finish their games, alot of leagues have had this problem over the years, and unless you make some really cool features like hall of fame and trophies it'll be hard to get the games done, espically if there are 2 sets of playoffs once a month and the same players reach both for free and classic.