Hello
You cannot define the time fence simply based on the production lead time. The value defined for the time fence depends on how fast you can react to changes on your plan.
If your plant can react fast to a change, maybe you don't even need to define a time fence. Otherwise, if a change on your production plan represents a high loss for your plant, you need to define a time fence to make sure that you will stick to your original production plan.
As already pointed by Solomon, you should discuss with your production planned, since this value cannot be defined purely by a technical perspective.
BR
Caetano