All Arete successes are counted as 2 for the purpose of being resisted. Only effects that directly alter the body and mind may be resisted.
For Example
Life Sphere - effects that alter ones physical body in any way may be resisted with an apposed stamina check (diff 6)
Mind Sphere - similarly, any effect that alters ones mind (IE Command/Emotional control) may be resisted with an apposed willpower check (diff 6)
Extended Arete rolls, or rituals may only be resisted by one roll for the player being effected. As in - the mage casts a ritual and accumulates 8 successes. The targeted player only gets one roll of stamina or willpower to get 16 successes (Have fun)
Sustained effects, such as keeping someone asleep by rolling every round, or continually doing damage, will be resisted each roll. IE - The mage is burning the player using fire, or keeping them asleep by rolling mind every round. Each round is resisted.
So, if a mage gets 2 successes on an Arete roll to cause fear in the character, that player may roll Willpower at difficulty 6. They must get 5 successes in order to shake off the fear (Mages win ties, sphere magick is that potent)