You can play for free for as long as you want. The downside being that none of your ships can increase their DarkLight by more than 2. Free players can still gain Epic Levels from enlivened Epic Blooms.
Payment removes the above restrictions. Paying for the game is a time based subscription - it lasts a certain number of months and has nothing to do with how often you play.
RipSpace costs £10 (UK pounds) and gives players 120 days of play. Players can play as often as they want during this time - the number of turns depends on how fast your enemies submit their turns. i.e. If players are only submitting turns every other day for combat in a given system then that is the rate of play for that system. On the other hand you may be in a system where turns are submitted by everyone every 30 minutes.