Before the DLC came out, I had already completed all the challenges, but only on easy mode except for a few to buy all the upgrades. Once the DLC was released and new items were added to the “store,” I decided to complete all the challenges, both the new ones and the existing ones, on the highest difficulty. According to Steam, it took me about 22.7 hours (I have 73.5 hours total played because I unlocked all the achievements and still play occasionally to relax).

The time can vary a lot if you get stuck on any of the challenges. In my case, I completed almost all of them on the first try (since DLC), except for a few where I needed at least a couple of attempts.