What would be the maximum time I should spend on doing that test?
You should spend no more than 1 hour, 48 minutes on the test. On a more serious note, that is a decision entirely for you to make based on how much you value your time and how much getting to the next round of interview matters to you.
Most annoying are the recruiters who try to explain to me why it's a necessary step in the hiring process.
There is an easy solution to this: simply stop engaging with such "annoying" recruiters.
How should I explain them when they leave it vague regarding the time spending on the task?
There is also nothing you need to explain here. You could simply tell them that you are willing to spend no more than a certain number of hours on their test. Be aware though, that they too could simply tell you they don't want to consider you further in the interview process.
While you probably have genuine reasons for your irritation with the pre-screening tests, you should also realize that the companies have their own reasons for those tests. You may not agree with those reasons, but if you want to work for such a company, you don't have much choice other than to follow their interview process. Of course, you could just restrict your job search to companies without tests that annoy you.