If someone is casting a spell with a casting time longer than a Standard Action, can somebody come up after the spell has been started but before it finishes and counterspell it? Does the answer change if you use Dispel Magic instead of the same spell the other guy is using?
For example, the BBEG is casting Legend Lore to find out something you don't want him to know. Sometime during the 2d6 weeks casting time, you bust in his door to stop him. For some reason you can't get to him in order to use one of the myriad other ways to force him to stop the spell (such as by punching him in the nose or whatever). Can your party spellcaster try to Counterspell the spell in progress?
The answer is "no" for using Legend Lore to counter it, but the
other ways of countering the spell would work just fine. In terms of RAW, Necro is right that it says you can only ready standard, move, or free actions. In terms of RAI it is a bit more complicated, because one might say the RAW simply did not take this sort of corner case into account: readying is about reacting, and couldn't someone react to a slow activity by initiating another slow activity?
The answer is that, even if one follows that logic, it is too late to start countering a long-casting-time spell with a similarly long-casting-time spell if you barged in after he'd already started. A readied action normally occurs before the action you're trying to prevent. For example, if
Black Spy is in a hostage negotiation with
White Spy, Black Spy might ready an action to counterspell, and as soon as he sees White Spy beginning the first motions of
summon bomb, he counters White Spy's
summon bomb spell with another
summon bomb spell. If Black Spy started casting his counterspell halfway through White Spy's attempt to summon a bomb, his counterspell would come too late.
Dispel magic is a different matter, as is
improved counterspell or a spell specifically designated as countering that spell. Since these can be completed before the enemy will complete his multi-round spell, he will counter the spell just fine.
Postscript: with a variable (2d6 week) casting time, it is arguable that you have a chance of casting Legend Lore faster than the enemy casts Legend Lore (for example, you can roll a 2 and he rolls a 6). Though un-RAW, I would allow it, though I would consider you awfully silly for doing so when you could just go home, take a nap, prepare new spells, and do something more sensible than that.