It really depends on what the content is for me. As a gamer, my content is always changing. I have a template for most of the elements I use on my thumbnails. The text and the image itself is usually the parts I struggle the most with. Finding the right image, editing it and coming up with an interesting title for the thumbnail text. It could be anywhere from 30 minutes to an hour. And this is just one thumbnail for a single video. If I make more than one to A/B test which one is better it can take twice as long.