First Step to Learn WordPress

WordPress is an open-source CMS, and it has many sections. You can either get a generalized education, or learn about each section, & become an expert in any one WordPress area.
Easy or hard, depends on your skills, and passion for learning WordPress.

It depends on what section you are learning and how much you want to know. There is no limit to learn WordPress, as it is forever evolving, and new technology emerges every day.
However, to start earning money from WordPress, you need to be expert in any section and have a showable portfolio.
But if you need quantitative measurement, then I will say 100 hours of serious learning is enough to learn anything.

WordPress powers the web, and not only this is a lucrative career, but it will keep growing over time.
Moreover, you acquire skills while learning WordPress that you can use on other platforms and jobs. Such as SEO, marketing, writing, speed optimizations etc., they are not bound to WordPress, but only a part of the ecosystem.
So, yes, WordPress is a good career.

Yes, WordPress have a safe future. In terms of competition, no other platform is even near WordPress.
As more and more people will go online, the business will need to sell on the web.
WordPress is the most affordable platform, and ideal for SMB, and even brands.

Skills depend on the WordPress section you are trying to learn.
I you want to enter WordPress development, then you should know PHP and a few other languages.
Similarly, for marketing and SEO, you have to be aware of abrupt changes made by Google algorithm.
Though you can acquire these skills quite easily and quickly, honing them will take some time.

You can put WordPress on the resume, and the specific expertise you have.
Many people become experts in speeding up the WordPress site, while others become expert in designing the site with WordPress site builders.