my understanding of it is that to some extent, each game is slightly different. so the best strategy might be to write down what plants you are selling and the prices you are charging, then click nursery, and keep track of what sells and how long it took.

Raise the prices of things that sold right away. Lower the prices of things that take too long to sell. Don't adjust your prices based on the sales of plants that have been butchered by pruning mistakes.

And keep in mind there are magic plants that, once discovered, can affect all this as well.

How long should something take to sell? It is up to you. How long do you want to be away from your computer? I think somewhere around 10-30 minutes is what is right for me.

Cheers,
-Borg