With pretty girls and Inner Sphere culture distracting them. Never.
Honestly, any Clan assigned to the Concordat would have stalled at least 50% earlier in their invasion in comparison to the canon attacks. We're talking about the people who (according to canon) basically pioneered interstellar human wave attacks. Plus, they haven't the slightest problem with unleashing nukes. Talk about a meat grinder. Love'em or hate'em, the Taurian Concordat is a tough mother when you're invading their turf.
That said, anyone pushing through the Magistracy would enjoy a walk in the park until they reached Andurien or Regulus. Neither one of those people are well known for playing nice with invaders either. Lets see, the Anduriens fought three protracted and bloody wars with the Capellans prior to the Star League...and we've already seen how Regulus responds to invaders.
Gibson anyone?