Where can i find the list of Max and Min execution time of wordpress plugin's and APIs?

It is very unlikely that you are going to be able to find this information, and if you do, it is likely to be very inaccurate given that execution time is dependent on many different factors that can't be controlled for across users / hosts / environments. However, 60 seconds is a generous time limit - the default for PHP is 30 seconds. Note that this limit is for how long the server takes to generate the response, not how long the page takes to load. For example, a website might take 1 second to execute PHP in response to a website GET request, but the browser might take 120 seconds to fully load the page that the server responds with. If your server takes longer than 60 seconds to process your PHP in response to just a normal web request, you very likely have other very serious issues.

It seems like the main reason why the Avada theme is asking for 300 seconds is for the ability to bulk load the demo content , which is probably doing something like downloading a ZIP file, unzipping it, and processing the content. This should only have to be done once, and you should be able to get around it by manually importing via FTP.

Typically this limit is only an issue for things like that; importing or exporting lots of large files, loading lots of content at one page (for example, if you set posts per page in wordpress to 1000), etc. Or bad code - a FOR() loop that never exits should have an execution time of "infinity".