When a caster takes an amount of damage over 0, there is a chance that the caster's spellcasting for the round will be disrupted. Use the following formula on each ROUND in which the character takes damage.

The spell disruption difficulty is defined in this manner:

10 + Spell Level - Caster Level/2 (round down) - Constitution /2 (round down) -5

spell disruption roll is defined in this manner:

Random 1-20

If spell disruption roll >= spell disruption difficulty, the spell is cast successfully. Otherwise, YOU BLEW IT.

Concentration is what is being checked. The idea is that an 18th level mage is so accustomed to casting 1st level spells that maintaining concentration over such simple magic under pressure is not difficult.
Once casters get a total of +14 to Concentration (skill ranks + con bonus + whatever). Defensive casting eliminates all ability to disrupt spellcasters unless you have a readied action, which almost never happens.

It all depends on the level of the spell being cast. However, a 1 never fails. If you're a 10th level mage (decent level, but not mind-blowing) you can have 13 ranks in Concentration and a 14 Con for a +15 to your roll. Casting a 1st level spell is automatic. Casting a 9th level spell would happen on a roll of 9 or more -- 55% of the time. Those are pretty good odds.

Casting Defensively: When spellcasters are near melee combat, they will automatically attempt to cast defensively. When they do this, they must make a successful Concentration check vs. a 15 + Spell Level DC. If they fail, they lose the spell. If they succeed, they cast the spell.