The important things here are how many characters per line you're getting and how many lines you have.
You cannot get an exact answer to this question because a page full of lower-case 'I's is going to represent more characters than a page full of capital 'W's. So at best your answer is only ever going to be approximate.
A quick Google search for "Characters per page" turned up this result:
According to that article:
at 10 points pitch you'll get 4972 characters per page (including spaces)
at 12 points pitch you'll get 3838 characters per page (including spaces)
Sooooooooo .... to fill 1MB ....
1 megabyte = 1024 kilobytes = 1048576 bytes, or the same number of characters (assuming one byte per character which may not be the case as they could be wide characters or Unicode characters).
Therefore, assuming 1 character is 1 byte we have
10 point: 1048576 / 4972 = 210.9 pages, call that 211 pages
12 point: 1048576 / 3838 = 273.21 pages, call that 274 pages
It's not an exact science, but if that article is about right then that's what you can look for.
Bear in mind though that word processor documents will contain other information - metadata - about your document, so you can easily have a 1MB document but not have as many printable characters on it.