For developing trees where I want strong growth I start a weekly aplication of water soluable fertilizer full strength starting in the spring as the trees break dormancy. When the summer temps stay at about 90 and above especially with elevated night time temps I stop fertilizing. Then I start again as the temperatures cool and the seasen progresses to fall. I stop as the trees go dormant (leaf drop) in late fall early winter. I alternate between organic and inorganic NPK sources. I tend to use whatever product or brand is on sale. I also add a few cakes (poo poo balls) as I see fit especially if I am not able to fertilize with the water soluable because of my schedule.
I should note that for the vast magority of my trees I use a totally inorganic free draining soil with a top dressing of spagnum moss. The soil is comprised mostly of pumice , lava and akadama in various combinations. I also have water that is moderatly hard and on the alkaline side of the PH scale. Ocasionally I will check the PH of the soil and adjust with an application of water soluable fertilizer intended for acid loving plants on an as needed basis. A Bi yearly repotting with fresh soil also corrects PH imbalance and disolved mineral and salt accumulation from the hard water.
For the few more developed trees in my collection where I want controlled growth I instead use fertilizer cakes. I make the cakes from mostly inorganic ingredients ie fish, bone meal, kelp meal cotton meal alfalfa meal, blood meal etc. Again I use what is on sale to form a cake or ball that is formulated from multiple ingredients to achieve somewhat balanced percentages of N, P & K. (A simple way to create the cakes is to purchase a formulated fertilizer comprised of inorganic source (Whitney Farms ) add fish for added stink and flour for binder.) I usualy dry the the fertilizer cakes for storage purposes. However, I find no harm in placing the cakes on the surface of the bonsai soil moist or even wet.
The number of cakes and the timing of the application depends on the tree species and the its stage of development.