Officially, interrupting the 1-hour of meditation or study results in no spells prepared...
Each of the classes you mention has no provision for partial preparation of spells.
- "Each cleric must choose a time when she must spend 1 hour each day in quiet contemplation or supplication to regain her daily allotment of spells."
- "A druid must spend 1 hour each day in a trance-like meditation on the mysteries of nature to regain her daily allotment of spells."
- "[A wizard] must choose and prepare his spells ahead of time by getting 8 hours of sleep and spending 1 hour studying his spellbook."
Using a strict reading of the rules as they are written, that's exactly how long preparing all of the appropriate caster's spells takes, and stopping early means no prepared spells. Yes, the day after the massive battle that exhausted all his spells, even Count Slimeheart, Archmage of the Drooling Tower, if interrupted 1 min. before finishing his hour of study, will find himself with no spells prepared.
But, as per Preparing Wizard Spells, a wizard
[p]reparing some smaller portion of his daily capacity takes a proportionally smaller amount of time, but always at least 15 minutes, the minimum time required to achieve the proper mental state.
Thus, a wizard can choose beforehand after resting to prepare a fraction of his spells, yet it's up to the DM if the wizard who planned to spend an hour preparing but actually spent less time preparing has prepared some of his spells or none at all.
Likewise, as per Preparing Divine Spells, divine spell casters who prepare spells must meditate for at least 15 min. at their appointed or chosen time, but can also leave spell slots unfilled for filling later, taking 15 min. per quarter of spells.
(A clever wizard or divine caster of prepared spells may start preparing 25% of his spells in 15-min. bursts instead of 1-hour long stretches, or the DM can just assume every experienced caster does this. I'm going to go out on a limb and say that this isn't the default method of spell preparation because the default method of 1 hour and get 'em all makes bookkeeping easier and arguments fewer.)
...And sorcerers need only 15 min. to concentrate...
Although sorcerers lack many problems their more devout and studious compatriots possess...
Unlike a wizard or a cleric, a sorcerer need not prepare her spells in advance. She can cast any spell she knows at any time, assuming she has not yet used up her spells per day for that spell level.
...the section Magic under Sorcerers and Bards says
Each day, sorcerers and bards must focus their minds on the task of casting their spells. A sorcerer or bard needs 8 hours of rest (just like a wizard), after which she spends 15 minutes concentrating. (A bard must sing, recite, or play an instrument of some kind while concentrating.) During this period, the sorcerer or bard readies her mind to cast her daily allotment of spells. Without such a period to refresh herself, the character does not regain the spell slots she used up the day before.
So there's a 15 min. window during which a sorcerer still hasn't regained yesterday's used spell slots upon awakening from 8 hours rest, but then the sorcerer's ready to go. (Of course, there's also the 50/50 chance he's 1 entire spell level below other full casters, but that's beside the point.)
...So the the GM could make a house rule that such concentration takes longer than 15 min.
If made at the campaign's beginning, a house rule mandating that the sorcerer (and similar casters like the bard) must spend, instead, an hour concentrating upon awakening from 8 hours rest is reasonable. Although such characters often have hard enough lives, such characters are also usually full casters, so it's not as damaging a house rule as, for example, a house rule that somehow nerfs melee attacks. It's a burden, certainly, but it's not an excessive burden so far as house rules go.
However, a GM who makes such a house rule mid-campaign then attacks the party of sorcerers 16 min. into their preparation time is really just asking his players to leave.
An Aside: Please don't attack PCs while they're preparing spells. If you do, they'll try to do the same to the bad guys. You don't want to spend a whole afternoon making a wizard baddie only to have the baddie dead before his turn because the PCs interrupted him while he was studying, so don't do that to the PCs your players have been tinkering with for months either.
If you're afraid of the PCs downing the big bad evil guy early, don't use him! Send instead paid assassins, bound or allied creatures, created minions, simulacra, projected images, or fanatics. Have the baddie mail threatening letters containing a magic mouth or explosive runes. While the PCs aren't around, have the villain explode their home towns, frame them for crimes, damage their reputations, hurt their moms, ruin their NPC friends, and eat their pets. Sending the BBEG into harm's way means the PCs should have a chance to end him. (And any method the GM uses to save the BBEG should eventually be accessible to the PCs!) If the only way to further the plot is a house rule that mandates sorcerers concentrate longer, it might be time to consider—sadly—a different plot.
