The purpose of terraforming a planet is to produce a sustainable planetary ecosystem, not the quick development of a civilisation. The timescales I use are equal to the age of the solar system (4.6 billion years). This is roughly the same time it took for life to develop on Earth, so it's an acceptable timescale.
I'm not suggesting a long-time scale for indigenous life to evolve from scratch, or even from bacteria. Terraforming a planet would involve importing Earth plants and bacteria and later, fungi and animals. The long-time scale would be needed to allow the planetary ecosystem time to stablise. Then, once it's sustainable, allow it to diversify. This would produce new species and sub-species more suited to the new planet. Don't forget, people are a part of nature, not seperate from it. If we, as an evolving species, want to live for aeons, we will need additional planets that will last just as long.