I plan to apply for an MS in CS in the United States as an international student. While some universities provide guidance on the Statement of Purpose word limit, I can see that most don't. In such a case if my Statement of Purpose is 1000-1200 words and ends up spanning 1.5-2 pages, can that be somewhat lengthy to read?
I would not exceed 1000 words. Honestly, you should avoid going over one page. 500-750 words is the range I would aim for. An MS in CS is not going to be a program that will be enthralled by lengthy prosaic statements of purpose.
As someone who has read many applications, brevity is much preferred. At the very least, a long statement of purpose gets skimmed, whereas a short one gets fully read. I have never seen a statement of purpose longer than one page that changed my mind towards acceptance because of its length.
Unless the university specifically asks for a longer statement of purpose, I would keep it somewhat short.
There is almost never a need for a statement of purpose to be this long. By aggressive editing -- rephrasing and deleting -- you should be able to cut the essay down to 800-900 words without sacrificing any actual content. The resulting essay will be better not only because shortness is inherently good, but also because removing the superfluous words produces a more mature writing style.
The trouble with saying "almost never" is that everyone thinks that they are the exception to the rule. But, most people who think this are wrong.