I don't have any concrete numbers and can offer only guesses to this. But in my experience, the game is pretty hesitant to change your tempo when the selection is "anytime." I vary my tempo all the time from game to game and season to season but rarely do I see a change at halftime. I think you need to be crushing someone in the second half for the game to slow you down. I would bet it's a sliding scale but it seems like being up by about 20+ with around 10 minutes to go is the sweet spot for a change to occur. Completely rough estimates but I would say maybe being up 40 at halftime, 30 with 15 mins left, 20 with 10 mins left etc. Likewise with speeding your tempo up.
For more control the "If Winning/Losing" options could be used but I've only used those a handful of times. Usually the tempo I choose was chosen for a reason. But obviously if anyone else has a better grasp of this or more concrete numbers that would be pretty interesting to learn.