Interesting question. I think several parameters govern the time required to finish the research paper such as time consumed to perform the experimental work, collecting data, interpreting the results..etc. So, it is difficult to give a precise time.
I agree it is difficult to say how long it takes to finish a research paper. In some ways, these are never finished. You just get to point to quit. The data gathering, compiling, and analysis are very time consuming, depending on the project. Sometimes, in my experience, the writing of the paper, making sure it is in a form to be published, is very difficult. You know what your research "says," but putting it in to "publishable words" is difficult.
Mohannad is right; it varies a lot depending on the project and person.
My experience. Sometimes you read a conference proposal and find that you have some ideas for that. Depending on the deadline, you may have 1-3 months for that. But never try to write on a completely unknown topic. Select one where you have some preliminary results or at least reading.
The presentation is not the full paper. If you will send it to a journal, you may end up in several rejections and revisions before it is published. So it may take several years. Even if you wrote what you wanted just in few months.
Some people are perfectionists, others are not. I often lose an interest to a topic in few months, and if it is not 90% finished by that time, it may be never finished.
No paper is perfect. Even one accepted by editors. The author decides what is worthwhile publishing. Sometimes waiting is a bad strategy, because the topic is hot today. In this case I put working papers online here. You may look and decide whether they are worthwhile reading.