This depends on a lot of variables but here's a few thumb rules:
1) We have daily maintenance and emergency maintenance. In daily you can decide how much time you keep the log files and how much time to keep indexes, in emergency you set a threshold of disk space and when it gets lower than the threshold the oldest logs and indexes will be deleted until enough data was deleted to go back over the threshold.
2) Without indexes we can still view the logs, but only opening manually one log file at a time. So if you set daily you need to ask yourself, how far back do I need to view logs and how far back I need to store them. Logs could be re-indexed if needed but indexing of a large number of log files can be resource draining for a time.
Either way I recommend setting retention that will keep you at least 10-15GB in the log partition free.
Kind regards, Amir Senn