Well the Workflow History list is hidden.
Where to find: But many times we want to go there and check our workflow.
So here is the link
You can also find this in SharePoint designer.
Open the site in designer
Navigate to All files
Click on Lists in All Files.
You will find Workflow History there
How long and how much gets Logged: By default, the logs are kept for 60 days.
To stop cleaning: If you want to keep the logs in history List for an longer time, you can do it by modifying settings of Timer job. To do so go to Central Administration –>Operations ->Global Configuration ->Timer Job Definition. Here is a msdn link https://technet.microsoft.com/en-us/library/cc298800.aspx