I started rendering an animation in cycles in the late after noon than went to bed with it rendering rendering at around 25 sec per frame (when tested at what should of been the most labor intensive frame) and it had 1718 frames. When you do the math this should take around 12 hours to render. When I got up in the morning 16 hours had passed and it was rendering at around 30 sec a frame. At 30 sec a frame it should only take around 14 hours but it was at 16 and was still not done (it was really close though). I’m freaking out thinking my pc over heated or something especially bc it felt slightly warm to the touch over the tempered glass closest to the cpu. Please help!
Render times are different for each frame, depending on the complexity of the content.
Any change inside the scene and within the image has to be computed separately.
Estimated rendering times are never accurate for the same reason, you can have a relatively simple scene to render at the beginning of the shot, and then have an explosion with particles and all kinds of complications or motion blur, those will take longer (sometimes much, much longer) to render.
3D Rendering is quite demanding in terms of resources and will push any computer. When computers get hot they go into a slower speed in order to protect the hardware. If the hardware reaches a critical temperature it will fail and shut down. If you experience abnormal heat and the computer is "throttling" you need to invest in some kind of cooling system.
I often had this same difficulty with animations. My camera was following a path and while the first frames render fast (10s) the later frames would take much longer (2min). The shown time of render is calculated only for the currently rendered frame, but when the complexity of the scene varies greatly over the animation also the rendertime will vary. The easiest way i know for making estimates of the total rendertime is to render every 100th frame or so and then multiply the time by 100