Well, I don't have experience with Subarus in particular, but generally it depends. Sometimes a coil pack will last for the life of the car, sometimes it will fail early on. My mother's Buick Century with 166k miles hasn't thrown a pack yet. So It really isn't something you can predict.
A little OT, but I would really like to hear about your MR2 project, as I own an 85. I assume you are already a member over at
www.board.mr2faq.com .