As a general rule it takes as little as 30 days to write 30000 words. Most professional fiction writers write 1000 words a day so a 30000 word novella will take 1 month to write the first draft, but not all that time is spent writing.
As a general rule, 30000 words in a book will make between 100 and 110 pages, depending on the font used. Each page has an average of 280 to 300 words. A book with 30000 words and 100 pages would be classed as a novella.