Use a field to track how much time was spent working on an issue.
Perhaps you need to keep track of time spent by your developers or support / help desk engineers for billing purposes. This can be done using a Floating Point or Integer field.
Prompt users to fill in the time they spent when they mark an issue as completed.
You can generate metrics with this data.
For more detail on how to create this field, check out the Track Time Spent Knowledge Base article.