SEO, How long should it take somone to write an excellent meta description?

For someone who already understands SEO, the significance of meta descriptions and the uniqueness of each description, using relevant keywords of course and keeping within the rules - 156 characters, no repeating keywords if not necessary, early and often, etc.

Please advise on how long you think would be a fair estimate to write 706 meta descriptions for 20 unique websites? The 706 figure is for a total of all 20 unique websites and not per site. Would it be like 100 meta descripts in an 8 hour period, 300? I just need a general type of time frame so I can let a company know. 2 weeks? 2 days? I've done it but have not had to estimate this way. Usually people ask me for keyword research, not to just churn out meta descripts. Any advice, guesses, etc. would be welcome. An actual reference would be greatly welcomed!