There is no "standard" time to 100 subscribers and it will be different for everyone. I've seen so many channels that have stayed in the low hundreds for months and then suddenly hit 1,000 subscribers overnight.
How did they achieve this? Quality content. There are no shortcuts, you just need...
Any CTR above 10% is already super impressive!
I like your thumbnail especially your expression. Potentially you could try a thicker border around yourself to make you stand out more from the rest of the thumbnail?