Quick numbers from importing my audit logs. They're in no way official predictions, but they should give you something to work with. My logs are compressed, so 4.7GB of logs go all the way back to Oct 2011. This repository is a test repository, so at times it sees tens of thousands of events a day, while at other times there is no activity, so data is not even distributed across the logs.

Importing the last 30 days of audit data, around 2000 events, gave me a SQL database with a 46 MB MDF and a 20 MB LDF. Expanding the date range to 180 days, 660,000 events, resulted in a 465 MB MDF and a 568 MB LDF. I then shrunk the LDF and it went back down to 1 MB. The 180-day range was covered by 6 logs, 14MB in overall size. At 365 days, 9 million events (53 logs, 72 MB), the MDF was around 8 GB and the LDF was around 800 MB. Manually shrinking the LDF got it back down to 1 MB.