Full round spell and being attacked

Question

If a caster casts a spell that in the casting time states "full round", or uses a metamagic feat casting and her spell casting time increases to "full round" is the caster "busy casting" until her next turn?

1. What happens if she is hit by a monster before her next turn?

2. What happens if she is hit by two attacks from the same monster before her next turn?

3. What happens if she is hit by two monsters before her next turn?

What I imagine is that in her round she needs to roll the dice for concentration, and then will have to roll concentration multiple times at every attack, but I may be wrong.

Answer 1

You have to distinguish between spells with a casting time of 1 round (or longer) and spells which are modified by a metamagic feat and cast by a spontaneous caster.

A spell with a casting time of 1 round (like, for example, Summon Monster) takes a full-round action to cast and "comes into effect just before the beginning of your turn in the round after you began casting the spell“. The caster is indeed "busy casting" until her next turn and has to keep concentrating on the spell. Spells with an even longer casting time (like, for example, Regenerate (casting time 3 rounds)) require the corresponding number of consecutive full-round actions to cast.

Any distraction (most commonly: taking damage) that causes the caster to make a Concentration check might lead to losing the spell. If she takes damage from several attacks, she makes one seperate check for each attack - even if the same opponent hits her twice.

Spells with a normal casting time of 1 action (or less) which are spontaneously modified by metamagic feats also take a full-round action to cast but come into effect right on your turn.

Casting a Metamagic Spell

Sorcerers and bards must take more time to cast a metamagic spell (one enhanced by a metamagic feat) than a regular spell. If a spell’s normal casting time is 1 standard action, casting a metamagic version of the spell is a full-round action for a sorcerer or bard. Note that this isn’t the same as a spell with a 1-round casting time—the spell takes effect in the same round that you begin casting, and you aren’t required to continue the invocations, gestures, and concentration until your next turn. For spells with a longer casting time, it takes an extra full-round action to cast the metamagic spell.

(emphasis mine)

Answer 2

Concentration

To cast a spell, you must concentrate. If something interrupts your concentration while you’re casting, you must make a Concentration check or lose the spell. The more distracting the interruption and the higher the level of the spell you are trying to cast, the higher the DC is. If you fail the check, you lose the spell just as if you had cast it to no effect.

Injury

If while trying to cast a spell you take damage, you must make a Concentration check (DC 10 + points of damage taken + the level of the spell you’re casting). If you fail the check, you lose the spell without effect. The interrupting event strikes during spellcasting if it comes between when you start and when you complete a spell (for a spell with a casting time of 1 full round or more) or if it comes in response to your casting the spell (such as an attack of opportunity provoked by the spell or a contingent attack, such as a readied action). [...]

1. She must make a Concentration check or lose the spell, DC 10 + damage + spell level.

Concentration (Con)

[...]

If more than one type of distraction is present, make a check for each one; any failed Concentration check indicates that the task is not completed.

[...]

Concentration DC	Distraction
10 + damage dealt	Damaged during the action. *
...	...

* Such as during the casting of a spell with a casting time of 1 round or more, or the execution of an activity that takes more than a single full-round action (such as Disable Device). Also, damage stemming from an attack of opportunity or readied attack made in response to the spell being cast (for spells with a casting time of 1 standard action) or the action being taken (for activities requiring no more than a full-round action).

2. She must make two Concentration checks or lose the spell.

• The first one at DC (10 + <first attack damage> + spell level)

• The second at DC (10 + <summed damage of both attacks> + spell level).

A kinder DM might interpret the rules to imply the second roll at DC (10 + <second attack damage> + spell level), instead.

3. See 2, the rules make no distinction between being hit twice by one attacker or once by each of two separate attackers.

Answer 3

Yes.

'If while trying to cast a spell you take damage, you must make a Concentration check (DC 10 + points of damage taken + the level of the spell you’re casting). If you fail the check, you lose the spell without effect. The interrupting event strikes during spellcasting if it comes between when you start and when you complete a spell (for a spell with a casting time of 1 full round or more) or if it comes in response to your casting the spell (such as an attack of opportunity provoked by the spell or a contingent attack, such as a readied action).'

https://www.d20srd.org/srd/magicOverview/castingSpells.htm#concentration

And.

'If more than one type of distraction is present, make a check for each one; any failed Concentration check indicates that the task is not completed.'

https://www.d20srd.org/srd/skills/concentration.htm