Feedbunch does NOT mark entries as read after a certain period. That's not a feature of RSS at all, I guess it is a feedly-specific feature (or anti-feature, I guess, depending on your needs).

What Feedbunch does, however, is limit how many entries it remembers for each feed. The current limit is 500 entries for a single feed; once a feed has more than 500 entries the oldest ones are removed until there are 500 left. The number (500) could be subject to change during the beta phase if users' feedback indicates it is too high or too low.