What's the best way to keep track of modeling time to improve customer billing?
Train your employees?
Other than that, there's not a way built in to SW. Any sort of automated method has its shortcomings as well. Even if you could track how long a model is open, the user might not be interacting with SW.
Welcome to SolidWorks forums Rich.
Some of the posts which might be of some help to you:
Tracking usage of Solidworks
Can anyone help me control editing time in assembly or in parts?
Time Management using API Macro
You might also find this post helpful for your future reference.
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