The IMAP spec only guarantees that IDLE connections can be alive for 30 minutes (and recommends subtracting another 5 minutes to be safe). So I'm not surprised about "30 minutes". Most servers will hold IDLE connections open longer, this is good.
The "6 minute refresh" in K9 Mail corresponds to the setting which you have at "30 minutes" in AquaMail -- a full closing and reopening of the connection.
The "10 minutes" (keep alive) setting in AquaMail corresponds to something which does not exist in K9 Mail.