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Filler (Bondo is a product name) will last forever, as long as you do the proper prep work. That means eliminating the rust before you apply filler.
Filler is not waterproof. That basicly means all holes in the metal have to be filled prior to the application of the filler.
PPG (a paint company) recommends you do your bodywork, apply epoxy primer, apply the filler, and then apply another coat of epoxy primer. The epoxy primer seals the filler.
If you're not equipped to paint with epoxy, you can apply filler directly to bare metal. It's been done that way for years.
Rage and Rage Gold by Evercoat are my 2 favorite fillers. Rage Gold seems to sand out better.
Rage gold was always my favorite in my shop days. Evercoat makes it and their products are quite good. Filler will last as long as the car provided you use it properly, don't mud the car like many people do. Rule of thumb is no more than 1/8 inch thick. If you are going to apply the filler to bare metal then make sure you run a grinder over it before. You don't want to start removing metal, but you do want the metal scuffed up so the filler will stick. Much like when you prep a car for paint.