So does it just happen? Is it a gradual change? Do we die out? Or do we evolve in a hundred years?
Yes it just happens. It is not directed or planned.
It is both sudden and gradual. The change happens suddenly. That change will be beneficial, harmful or neutral. If it is beneficial, for example it allows access to a new food source or movement into a new ecological nitch, then those animals that have that new ability will gradually become a different species. But the change was sudden. The reproduction until there are sufficient numbers to become a seperate population will take time.
If it is harmful, then that group will gradually die out.
Most of the changes will be neutral, for example eye color. In those cases the change will stick around.
And evolution can occur in brief periods, particularly during times of great population stress or sudden enviromental changes. Some good examples are the variety of birds found on isolated islands where species have rapidly evolved to fill the myrid of possible habitats. But it can also be slow like the evolution of the different species of Homo.
Aslan is not a Tame Lion