How often do the sheets need to be replaced? Or what signs do I need to look for?
Thanks
They can be used for as long as you keep getting good results. Weeks. Months.
If you can keep yourself from tearing, cutting, or gouging the PET when you remove parts, the sheet should last a long time.
I would say if you are tearing up the pet sheet. Your being a litte too rough when you remove your objects or your first layer height is off and you bed isn’t properly calibrated. I had one of those green sheets last a year. It only failed when I didn’t properly level the bed.