Does anyone know when the construction cost discount time is?

I'm level 21 and the hidden quest is around level 31 or 35. Since the time limit doesn't seem easy, I'm aiming for a construction cost discount period, but I don't know when that will be. It's a 10% discount for the most expensive building, but up to 50% for cheaper ones... If I use what I've diligently saved, I think I can clear about 2 stages.