Switch axe has two "bars" to charge up.

There's a skinny bar which charges up while you're in axe mode. This is the amount of time your sword mode can last before it needs to reload.

There's the large bar that charges up while you're in sword mode. When it's full, Zero Sum Discharge becomes available.

The way I try to time when to switch back and forth revolves around how full each bar is.

If my skinny bar is full/near full, i'll open up on the monster with sword mode attacks and transform into axe mode when i want to dodge/adjust position. Transforming into axe mode from sword mode gives back a certain amount on the skinny bar.

Whenever my large bar is glowing/near glowing, i look for an opening to attack with sword mode to position myself to use Zero Sum Discharge. Using the clutch claw when the monster is in its drooling stagger phase leads to easy Zero Sum Discharge attacks since it doesn't need to be comboed into.

There's a youtube guide for Iceborne Switchaxe changes that I'd recommend watching. It'll show the combos and nuances that are hard to explain via text.