Are the challenges always at the same times?

Yes. The challenges are launched during periods of cold weather, during high demand periods of the day: mornings, from 4 a.m. to 11 a.m. (4 a.m. to 6 a.m. for the preheating phase, 6 a.m. to 10 a.m. for the reduction phase and 10 a.m. to 11 a.m. for the recovery phase) and evenings, from 3 p.m. to 10 p.m. (3 p.m. to 5 p.m. for the preheating phase, 5 p.m. to 9 p.m. for the reduction phase and 9 p.m. to 10 p.m. for the recovery phase).

This article outlines the differences between the different device types included in a challenge.