WhaleShark Media announced the acquisition of RetailMeNot, in a deal that is understood to be worth close to $90 million. RetailMeNot itself will make close to $30 million this year in revenue, with what is believed to be a high profit margin.

from here

My usual answer is ASAP, which is actually correct.  The best time is now when the economy is in a recession more and more people are starting to look for coupons online instead of driving to the nearest mall.

But …There is also a thing called “Winter holydays shopping” which happened every November-December. If you start your Deals/Coupons website right now you still have six month till November to promote your site.  Six month is enough time to put some good content,  become well indexed in all major search engines and do some link building to improve you ranks.  This is the time that you need to bring your site to its full power, so you can make some really good money on this “Winter holydays shopping” spree.

So, if you are still waiting for the right time, this time is NOW!