The default retention for 'Events' is 30 days. I believe you can still find this under the "Settings" -> "Polling Settings" and increase the "Events Retention" from the default of 30 to whatever you desire.
Per your posted question, you can retain as many days that you need. Though your database will increase in size when you increase the # of days of events retained, just be sure to watch your DB size and associated resources. This setting may be determined by your current SOP, management or other governing policy. If it is up to you to set then have at it, I have never gone passed 120 days with this setting, but usually settle between 65-95 days for 2-3 full months of retention.