Don't see a perfect fit? Please let us know.
What happens when the limit is reached? It's up to you. Logs can stop or can continue as a usage-based service (up to 200% extra, at 20% higher price per GB). You're always notified and can switch at any time.
How did you choose these durations? We used them. 3-7 days satisfied something like 97% of our own searches. Early users made the same observation. Archives worked well for the rest.
How is log transfer (GB) calculated? The length of the message, plus 50 bytes for metadata. 1 GB is 6.6 million 100-character messages. Transfer is measured monthly, so day-to-day variations are fine.