How to use the "Switching characteristics" parameters in the BJT's datasheet
Is there some simple math equations, or estimations to get them from
  the given ones at given conditions?


On occasions like this I would immediately turn to LTSpice or whatever spice simulation software was available and look how fast it is going to operate under the supply and load conditions you have. The formulas and equations held/implied within the spice models will be more accurate at predicting real life than "simple math equations".

If you feel that a simulator is too big a step, I'd still encourage you to go down that path because they help me to solve a lot of engineering problems and give great results. For what you want here, they would give +95% accurate values I reckon.