Depends.

Homeworlds are considered nearly impossible to take because their race has been on that planet for thousands upon thousands of years. There is a settlements and a military facility everywhere and contending with an entire planet and the total fleet power of that empire if they are capable. Such an invasion requires a logistical and military operation that can take years to even organise, taking that world will take longer. It will cost billions of lives as they have to take every single facility and settlement. Many just seek to destroy the worlds and take their resources to benefit their empire.

Core worlds are very hard to take, a handfull of fleets and a giant invasion force can take any big settlements and take a few years to fully conquer the world. There will always be a resistance somewhere in the wilderness and can take some work to get fully rid off.

Colony worlds are often the one that is taken in wars as they are the easiest, they are extremely common and the planet is conquered in a handful of battles and doesn't require a lot of manpower to take. Campaigns can take longer or shorter depending on resistance or even the presence of other colonies on that planet.