It also depends on the nature of topic, time management by overcoming known and unknown hurdles and obviously the researcher's love and addiction for the work. I think a Ph.D thesis can be completed in more or less 3 years.
I took four and a half years to complete my Ph.D work because I did as a staff-member of Botanical Survey of India.
Here I am assuming that thesis means Ph.D thesis but in my circle I have seen no one to complete in just 6 months. For M.Sc. thesis said time period stands good.
If you mean writing a thesis only ,the time required for not lasting more than 6 months as average,if you mean the time for all a thesis requirements then it depend of nature of topics ,for example agricultural studies required a field experiment for one season at least ,followed by chemical and statistical analysis lasts for 6 months at least ,and finally writing a thesis which required many months .
As others say it depends on the topic, the institution, whether you are fully funded (or not) and what has been done before. In New Zealand the intended time for an M.Sc. is now usually two years - one year for papers and "in theory" one year for the research and write up - in effect most of the research is done in 6 months or less allowing for more time to achieve the write up. M.Sc. used to take three years (or rather have a maximum of three years). For a PhD the intention is three years (full time) and often there are financial encouragements put in place to ensure a student does take only three years but the average here is still five years, and many PhD are done part-time anyway (mine took eight years and know of others than took 10). Of course the type of dissertation output is also critical for a PhD as some institutions favor publishing your research and then binding up the papers as a thesis, while others prefer a thesis from which you can do both - I did a mix of these. If you are an established researcher a sensible move is to consider a D.Sc. - whereby you can bind up your best papers into themes, submit that to the institution where you got your last degree and see if you are successful...
It depends on the style of thesis. As a monograph the duration of the writing is typically about half the duration of the data collection and analysis component of the project. If the thesis is presented as a collection of published and submitted journal articles, then the students I have supervised have taken 1 to 1.5 years per article. Depending on the capabilities of the student, in my opinion most are likely to finish more quickly with a monograph style.
I agree with Willem. My thesis consisted of a collection of published and submitted journal articles. Each took 1-1.5 years from submission to publication. However, I only spent three months on the final thesis write-up to link all articles together.
By the MS Word gives a measure of total editing time/document, I find it a good measure for actual time you spent on putting down your work and reviews. For my work it took me an average of 30 min/per page.