As Hakan said, if the log is growing too fast, there must be something abnormal going on, and it is worth a look.
When full time cruising in 2012, I had to hit "OK" for the log.log message on my PC just 6 times (I always try to keep all history), so once about every 2 months. We had 2 or 3 laptops on board, so let's assume it was once a month on some of these ... I would think this is acceptable. The log definitely does not include data about AIS targets.
The log is most easily viewed by clicking the blue question mark "Help" icon, then "Copy log to clipboard", then start some editor and "Paste". I have a shortcut link on the desktop
to opencpn.log file directly, so it opens in Notepad immediately ...
Of course, it is possible and easy to implement an option to handle the logging in silent way, e.g. alternating between two files. It would not be wise to let it grow uncontrollably, or drop logging altogether... A Feature Request?