If you are planning to write a good quality, meaningful, business plan then rather than posing this question, just turn it around and ask "How much time should I spend on writing a business plan?"
Some of the items in the business plan document are routine (name, address, contact number etc.). So if you so like, finish off this part first and have a sense of achievement. Then turn to the other parts.
The most important purpose of a business plan, according to me, is to face the inconvenient questions that one has been dodging or pushing under the carpet. By dismissing them as irrelevant, inconsequential etc. he tries to escape from the unpleasant task of collecting the facts, interpreting them, doing the research, cross-checking, etc. It is a hard work. That's one reason one tends to avoid it. The other reason is the lurking fear that if things don't fit together, one may have to modify or even abandon his business idea - an idea that he has been holding so close to his heart, has been deeply in love with and is desperate to implement as it is. This could be quite dangerous.
My suggestion: Just start compiling one. Download a template and start filling it up. You will get stuck. Leave it there, and go to the next section. Go on till you reach the end of the document. Then start over. This time select one of them and put your nose to the grind. Do it really well. Top quality stuff. Don't worry about the time you are spending on it. It is an investment. Be honest. Don't gloss over tough points.
That's the best way to go about it.
All the best.