So the Schlieffen plan was never truly followed. It's important to note that the plan failed. We have to then ask whether or not the time was the reason.
Part of the problem obviously comes from the violation of Belgian and Dutch neutrality for no reason other than military expediency. The political consequences of this were huge, and Schlieffen didn't expect them or plan for them. He simply figured they had to do it, to make the plan work. People blamed von Moltke for the plan failing, saying he should've violated Dutch neutrality. However, he managed to stick to the plan without doing so, and the Dutch neutrality ended up actually helping the Germans in the grand scheme of things. So really, the issue wasn't that he didn't violate Dutch neutrality; the problems were far more widespread than that. Schlieffen acknowledged that the right wing would have a very hard time keeping to the plan.
He underestimated just how hard a time that would be.
The wide sweep and mass of the right wing had huge logistical and supply problems, which made the timetable extraordinarily unrealistic. He didn't consider that the French and Belgians would destroy the railways that he expected to be able to use, which they did. The soldiers were stuck marching on foot, reaching the Marne exhausted, while the French brought in troops by train. Every vehicle in the German army wasn't enough to solve these problems, and the timetables got all kinds of screwed up as a result. Schlieffen had just counted on pure audacity, and when the plan failed, von Moltke was blamed for not having that "audacity".
Holger H. Herwig posits that the timetable was horrific, and contributed to the plan's failure. He argues that the timetable was far too restrictive, didn't account for any part of the plan's failure, and was essentially the downfall of the German forces in the attempt to implement the plan.
Terence M. Holmes, on the other hand, criticizes this. He actually argues that the timetable was never set up rigidly. He says the "42 day constraint" that Herwig and others refer to appears nowhere in the actual Schlieffen plan. Where Herwig got that information, Holmes is unsure. It seems to be a huge source of contention. The only real indication of it is that the documents that Herwig says he's citing also apparently said that after 40 days the Russian troops would begin their attack, and the western front would have to be wrapped up by then.
However, even if this were authentic, as many seem to believe, there's plenty of reason to believe that the timetable was poorly designed as too rigid. It was assumed to be the perfect plan, and many sources I've seen mention that hardly any other alternative plans were drawn up. It was simply regarded as a work of masterpiece in war planning, and the timetable was both too rigid and too unrealistic.
That's hindsight, anyways. With the information the German command was privy to...I mean, they could've factored in the destruction of those rail lines, they could've factored in the logistical issues that would arise, but they probably wouldn't have been able to implement the plan anyways if they had because there would've been huge logistical problems even still. They knew how wide the sweep would be, how difficult it would be to feed them all, but assumed it would work, just as Schlieffen acknowledged in his plan. He said a few things about how the right flank would suffer troubles. He said "...the right wing must make very great exertions" and that "a war of aggression calls for much strength and also consumes much and that this strength dwindles constantly while the defender's increases..." Even so, he still imposed impossible tasks on the right flank.
Some people said, not long after the failure, that if von Moltke had just increased the strength of the right flank, or changed the trajectory of the swing a little bit, then it would've all been fine. This is just another attempt to blame Moltke for the failure of the plan. As Liddell Hart suggested "it is useless to multiply numbers if they cannot be fed or provisioned". It is likely that any trajectory change would've altered the plan considerably, and the spread of the troops still would've led to the logistical nightmare.
All in all, it was a great plan for an ideal world. But with the destruction of the rail lines, the imposition of a slightly unrealistic timetable, and the issues with keeping the right flank working according to the plan (since it was so insanely difficult), it was not a really realistic timetable. The Germans thought it would be, but many suggest this was just wishful thinking. Whether or not it was, I can't confirm, but that's how it seems to me.
Sources:
The Schlieffen Plan and Plan XVII: A Short Critique Philip M. Flammer Military Affairs, Vol. 30, No. 4 (Winter, 1966-1967), pp. 207-212
"One Throw of the Gambler's Dice": A Comment on Holger Herwig's View of the Schlieffen Plan Terence M. Holmes The Journal of Military History, Vol. 67, No. 2 (Apr., 2003), pp. 513-516