I am a late comer to programming, how much time do I need to spend in order to get a junior web developer job?

You need to spend approximately the following:

• 1 HTML5
• 1 CSS3
• 1 CSS Framework (Bootstrap, Material something or other)
• 1 JavaScript (ES5)
• 1 Version Control (just go with git)
• 1 jQuery (optional, unless you're gonna learn Angular)
• 1 JS Framework (Angular, React, Ember, etc.)
• 2 API implementations

EDIT: This is a recommendation based on getting a job as a junior developerand not a holistic approach to becoming a well-rounded developer.

Everyone differs considerably when it comes to learning - both in terms of the amount of time it takes them to grasp a concept, the amount of time they can dedicate to learning per some arbitrary period of time, and the quality of learning resources they have at their disposal.

I would say learning all of these could take anywhere from two weeks (with a really good teacher who has a solid curriculum and can give you their full attention for 80 hours over that period with you spending the full two weeks learning - more than just 8 hour days) to a few years (because you didn't know what to learn, fixated on the wrong details, didn't have a mentor to guide you in the right direction, didn't put education into practice to solidify learning, etc).