Generally, you keep beer in bulk until it is ready. That means if you want to use a secondary, you should transfer just before it is completely fermented. That allows for the still active yeast to form a protective CO2 layer over the top of the beer.
Much easier and safer from an oxidation/contamination risk perspective though to simply leave it in primary.
Regardless of if you use a secondary or not, the beer is ready to bottle or keg when it clears. How much time it takes to clear varies a lot depending on the beer, but usually 2-3 weeks after fermentation finishes is plenty of time for an ale. If in doubt and using a kit, just follow the directions. Leaving it a little longer is usually fine, cutting things short typically is not.
Adding things like cacao nibs, dry hops, fruit, oak chips, etc. is usually the only time I will use a secondary, but that is probably just the last stage of my evolution from secondary for everything to eventually eliminating them entirely.