1. The wording says "Once per day, an eidolon can become incorporeal for 1 round per summoner level." does that mean i can CHOOSE for it to last UP-TO 16 turns? or is my Eidolon FORCED to stay incorporeal for 16 turns
The ability says one round per summoner level, and does not specify a way to turn it off early. You must wait out the entire duration.
2. if it is 1 turn, will he be incorporeal until the end of his turn (I.E until the person after me acts), or until the next time the Eidolon has to act?
Not applicable, see 1.
3. is the activation\turning into incorporeal a free action?
Spell-like abilities are standard actions unless otherwise specified. This doesn't otherwise specify, so it is a standard action.
4. how does damage/armor work in this form? Incorporeal creatures have no STR and Natural Armor. so my huge 46 STR/54 AC Eidolon does zero damage to corporal creatures?
OR maybe the eidolon doesnt even "Become" incoporeal, but only gains its qualities and subtypes. which means he simply effected by:
http://www.d20pfsrd.com/gamemastering/conditions#TOC-Incorporeal
and
http://www.d20pfsrd.com/bestiary/rules-for-monsters/creature-types#TOC-Inco rporeal
It can attack, but it uses its Dexterity modifier to adjust attack rolls and does not apply anything to its damage rolls. However, unless its attacks were magic, it could not deal damage to corporeal targets, and even magic attacks deal only half damage if the eidolon's weapon doesn't have ghost touch.
As for armor, it loses all natural armor bonus and gains a deflection bonus. If it has an armor bonus, this applies only against attacks by other incorporeal enemies.
Becoming incorporeal and gaining the incorporeal special quality are basically the same thing.
1:as per
Incorporeal Form (Sp) Once per day, an eidolon can become incorporeal for 1 round per summoner level.
Spell-like abilities, as the name implies, are magical abilities that are very much like spells. Spell-like abilities are subject to spell resistance and dispel magic. They do not function in areas where magic is suppressed or negated (such as an antimagic field). Spell-like abilities can be dispelled but they cannot be counterspelled or used to counterspell.
Many durations are measured in rounds, minutes, hours, or other increments. When the time is up, the magic goes away and the spell ends. If a spell's duration is variable, the duration is rolled secretly so the caster doesn't know how long the spell will last.
If the duration line ends with "(D)," you can dismiss the spell at will. You must be within range of the spell's effect and must speak words of dismissal, which are usually a modified form of the spell's verbal component. If the spell has no verbal component, you can dismiss the effect with a gesture. Dismissing a spell is a standard action that does not provoke attacks of opportunity.
The ability lasts the hole duration because there is no "D"
2: see 1
3:as per
Most spells have a casting time of 1 standard action. Others take 1 round or more, while a few require only a swift action.
It has a casting time of 1 standard action, because it doesn't say otherwise.
4: as per
An incorporeal creature has no physical body.
It is immune to all nonmagical attack forms. Even when hit by spells or magic weapons, it takes only half damage from a corporeal source.
An incorporeal creature has no natural armor bonus but has a deflection bonus equal to its Charisma bonus (always at least +1, even if the creature’s Charisma score does not normally provide a bonus).
It has no Strength score, so its Dexterity modifier applies to its melee attacks, ranged attacks, and CMB.
Likewise, its spells or spell-like abilities deal only half damage to corporeal creatures.
The Incorporeal Eidolon dos no physical damage to corporeal creatures unless it has Magic Attacks (Su), then it doe 1/2 damage, using Dex instead of Str.
It's AC is 10 + Charisma Bonus + Dexterity Bonus + Size Bonus.Edit: Ninjaed by Saethori