Rather than rant about the time and finesse it takes to create an atmosphere and a living ecology, I'm just going to skip to the pros and cons of having terraforming take centuries vs. those of it being possible within the RP.
If terraforming takes centuries, that means the focus is on finding and coveting habitable worlds, not creating new ones. Each one is special, unique, and irreplaceable within the RP's timescale. If a race loses their homeworld, they have to find and adapt to a substitute, not just create a new copy of the old one. When someone finds a new world that already has life, they have to survey and study it, not just reforge it.
Races that have been stable and had advanced tech for centuries will have more habitable worlds in their core spheres than fast-growing empires. Less advanced ones could still manage, if they'd had thousands of years. Sure enough, they don't have many other advantages in SARP, so this would be a nice one to have.
If terraforming takes years or less, players can feel they're making an impact in real-time, and see all stages of the process without having to visit works in progress. The chaos of the land rush might create interesting plots, though we'd really have to ignore all the NPCs who would be doing this on thousands of planets at all times. The setting would logically be overwhelmed with habitable worlds in a decade, so logic would have to be ignored.
Note that creating artificial habitats can already plausibly be done in the same length of time--years or less--and with very much similar consequences. Players don't normally use a whole planet, anyway, so making a single dome city or orbital station might be a more reasonable project.