The larger the log the more performance issues you will have. If you have meta type changes enabled your event log will get very large very quick. During development, I will set it to 20,000 to get a history of what's going on but then reduce it to 7000 or less and turn off logging meta changes.
So I don't believe there is a max but know the more data you have in there the slower it will be. I had a client a few months back that didn't manage their data and their database was over 150GB in size and their event log couldn't keep up because of so many other errors with nearly every request and they had their event log set to 50,000. Needless to say, keeping your database clean is better than gumming it up.