I have looked at the same question for our IoT balloon project. Found an explanation that 30 seconds daily sending time translates into 500 packets of 10 bytes a day at the normal SF7. An average of 3 minutes between messages.
For different message size, you can rescale but take into account the extra 14 byte overhead for each message.
As I understand, for different SF, each step up makes the message send time twice as long. Which means 6 minutes average between SF8 messages, 12 min for SF9, 24 min for SF10.
To ensure the balloon remains visible we like some stronger messages as well. I decided for a sequence of 8 normal SF7 messages and added one SF9 and one SF10. My guess is this adds up to 8x 3 min + 1x 12 min +1x 24 min, thus the sequence can be sent 24/7 with 5 mins interval between messages.
Does this make sense?
Does anyone know if the 30sec per day policy is enforced?