How much time do you estimate to course maintenance and learning the platform (Storyline)?

With time estimates of this sorts comes assumptions.
Assumptions:

• Knows powerpoint well.
• Have never used any e-learning authoring tool, knows nothing about e-learning production.
  
  Given those assumptions id say that going through the basics course as linked above takes about the three days you say,
  Then...and estimating this is guessing in the extreme..
• taking that new knowledge and start to try it out in Storyline 360
• setting up a small live, relevant project
• Start to use templates available in storyline. 
• Get to understand how to use the slides/scenes available in the templates
• Then gather / create content for that project
• Populate the templates with content
• review , a process..
• learn about the ways to publish
• Publish and evaluate the user response on multiple type devices.

At this point the person have done their first e-learning. But the assumptions really have not changed other then that they have done their first e-learning.

If its a good e-learning that solves the previously identified learning gap, is not up to learning the templates and populating the templates.

But lets say this process as described completely ad hoc... in a small but relevant project, takes a couple of weeks. Somewhere around 80-160 hours depending on project type, prior skillset and support on reusing /repurposing existing content from the company.

Based on your description, I would rather consider to stay in powerpoint for much content creation and export images etc.. and Camtasia for good software video recording and editing and Rise360 to put it together with all the other content, including interactive sections in your course and creating quizzes for completion measurements in rise. Then publish for use on multiple devices.

Still roughly the same time frame for the project, but you give rise about a few hours to learn the basics and a couple of days to Camtasia basics. Then the rest to the project. 

When you need deeper tracking of the course participants results, or you want to create a custom interactive experience learning your products. Then go Storyline 360.

That would be my guidance to someone new to this area. To start producing content.
If it s good or not for the target group. will only the users be able to say for this type of production made by a complete beginner.

Hope this helps.