No one can say! The IVA companies are a law unto themselves. We owed �62k, we paid off �13k in the first year and offered a full and final of �30k. The IVA company wouldn't even put the offer on the table! A few months later my OH was changing jobs and losing part of his pay. We offered �40k and they accepted. As it turned out my OH didn't lose as much as we thought he was going to and, as a result, we are much better off financially.
Looking on one of the many debt forums, it seems if you offer a figure close to what you would pay if it ran for the five years, it may be accepted. They would rather have the money now than wait for another 2 or 3 years. Hope this helps.