How quick is an immediate action (spell)? [duplicate]

Question

Immediate actions "can be performed at any time", but how quickly can they be used?

Does it mean a spell like emergency force sphere could be used to avoid (for instance) fireball?

Ie

1. Wizard Bob sees his arch rival Wizard Zob; at long distance, initiative time!
2. Wizard Zob wins initiative and several rounds go by of mage battle.
3. Wizard Zob's next round, he decides it's fireball time!
4. Bob thinks fast and casts emergency force sphere around himself to stop the fireball.

Can Bob do that?

Looking for answers primarily for Pathfinder, but answer for 3.5 as a bonus (if it's different)

Answer 1

Yes, as long as not flat-footed.

Immediate Action

an immediate action can be performed at any time—even if it's not your turn.

You also cannot use an immediate action if you are flat-footed.

Emergency Force Sphere

Casting Time: 1 immediate action

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According to Condition #2: Yes.

Since spells, such as fireball, do not go through a wall of force, it would simply damage the force effect as normal.

Emergency Force Sphere

As wall of force, except you create a hemispherical dome of force with hardness 20 and a number of hit points equal to 10 per caster level.

Wall of Force

A wall of force can be damaged by spells as normal, except for disintegrate, which automatically destroys it.

Breath weapons and spells cannot pass through a wall of force in either direction, although dimension door, teleport, and similar effects can bypass the barrier.