Lets assume following scenario: I have Github Actions workflow in my private repository which runs two parallel jobs, each of them takes 5 minute to complete, so workflow itself finishes in 5 minutes.
Do I pay for 5 minutes (that's how long my workflow was running)? Or do I pay for 10 minutes (that's sum of my parallel jobs execution time)?
You would pay for 10 minutes because you used 10 minutes of machine time. The pricing is different by OS so you would also need to take that into account.
How are the minutes calculated? The pricing details at the bottom of https://github.com/features/actions are missing some detail. If my job runs for 1 second, does it get billed as 1 minute?
another related question: are the free minutes per repo or per organization? For example, Pro plan includes 3000 minutes per month. Can I use 3000 minutes for no additionnal cost every month in each of the 5 repos of my organization?
Can I use 3000 minutes for no additionnal cost every month in each of the 5 repos of my organization?
The way I understand it is you get 3.000 minutes with your Pro plan, not with your repositories. So, no you wouldn't get 15.000 minutes over 5 repositories.
Additionally, a Pro plan is a personal plan, not an organization plan. So if you as an individual with a Pro plan join an organization and want to run actions under that organization, the organization would need to have a Team or Enterprise plan (or have public open-source repositories) to be able to use Actions. Your Pro plan only allows you for 3000 minutes on your own personal repositories.