A good time to sell is when you are ready.
The market will be the market weather you sell or not. The focus should be on what your need s are and in what time frame you would like to meet those needs. For example, it may be a declining market but you have a job opportunity that in the long run will benefit you and this opportunity may not come again. Or, you may not get top dollar today if you sell but you are buying at a much lower price and your property taxes along with low interest rates will all help you get that bigger home you want at a lower monthly payment and lower down payment. As long as the Realtor you choose gives you accurate market information to make a wise decision you will be fine.
So remember, the best time to sell is always when you are ready and not the market.