I used your services to apply for my first 5 year period with the 30% ruling.
My period expires AFTER January 1, 2012 and I fulfil all the new requirements for the rule. Will I be able to still apply for another 5 years (10 in total) or just another 3 (8 in total)?
The reduction of the maximum term of the 30% ruling from 10 to 8 years will only affect the new applications. This is explicitly stated in the law. You already have the 30% ruling, so nothing will change for you as long as you fulfil the requirements.
Besides that the declaration you received from the tax authorities mentions the start and end date, based on the maximum term of 10 years. This was an official decision from the tax authorities, it can't be changed afterwards just because the government decides to reduce the maximum term on a later date.