the lord of the rings - Why did it take Sauron so long to regain strength?

According to the timeline of the Third age it took Sauron roughly 2460 years to regain strength and return to Middle Earth.

Why did it take him that long? And if he "returns" to Middle Earth, where did he spend the two and a half millenias in between?

Furthermore: Where did the additional strength come from? Obviously not from the One Ring, since it was lost or (as far as Sauron knew) destroyed. If this strength has been with him all the time, why did he not draw from it when Isildur cut the Ring from his Hand?

Is there any Information about this in any of Tolkiens Texts?

Answer

The reason it took so long for Sauron to "return" (considering he never really left Middle Earth) was because he had put most of his power, strength and will into The One Ring. This meant that even though his physical form was destroyed he would still exist.

http://lotr.wikia.com/wiki/One_Ring

The Ring of Power was created by Sauron. They are one. Sauron survived because the Ring did.

Even when his physical body is destroyed it doesn't mean he loses all his power. This is demonstrated when Sauron is drowned by Eru after destroying Númenor; he was able to move The One Ring even without a body. Then his body was destroyed a second time as well.

http://lotr.wikia.com/wiki/Sauron

Despite his defeat, Sauron was not vanquished permanently. Though greatly weakened, and in non-corporeal form, he still existed, due to pouring most of his native power, strength, and will into the One Ring. Thus, as long as it existed, he could never be truly defeated, and during the first thousand years of the Third Age, he lay in hiding, slowly recovering his strength until he was once again able to create a body for himself.

He had already gained enough power around the first millennium of the third age and was disguised as the Necromancer at the Dol Goldur. Gandalf after failing a first attempt at finding out if the Dark Sorcerer was Sauron in TA 2063, successfully discovered that Sauron had returned during a second intrusion at Dol Goldur in TA 2850.

Gandalf the Grey made a second intrusion into Dol Guldur in TA 2850, and finally discovered that the Necromancer was indeed Sauron. Eventually, the White Council put forth their might and drove Sauron from Dol Guldur permanently in TA 2941. Without the Ring in his possession, Sauron could draw on only the smallest fraction of its strength, so that his enemies were able to drive him from Dol Guldur with relative ease.

Essentially the problem with Sauron is that he put most of his essence into The One Ring, he still had powers that rivaled most mortals in Middle Earth while The One Ring existed. Plus he was a Maia which was one of the Ainur.

The below quote is from http://oakroadsystems.com/genl/ringfaq.htm#Q1-ReturnFromNumenor It speaks about how Sauron was able to move the One Ring after his physical body was destroyed at Numenor the first time when Eru directly intervened and Númenor was drowned under the sea. L#211 is one of the Letters from Tolkien as described in the bottom of that website.

Tolkien simply brushes the difficulty aside: “Though reduced to ‘a spirit of hatred borne on a dark wind’, I do not think one need boggle at this spirit carrying off the One Ring” back to Middle-earth after the drowning of Númenor. [L #211 (280)] The solution is that Sauron was a Maia, one of the original Ainur. The Ainur had the power to manipulate the physical world by pure effort of will; that’s how they finished shaping Arda after entering into it in its “raw” state after creation. We know that Sauron in particular had this ability because he created a body for himself several times.

It's stated by Gandalf at his "final death" that the power he had originally had was gone now.

"If [the Ring] it is destroyed, then he will fall, and his fall will be so low that none can foresee his arising ever again. For he will lose the best part of the strength that was native to him in his beginning, and all that was made or begun with that power will crumble, and he will be maimed for ever, becoming a mere spirit of malice that gnaws itself in the shadows, but cannot again grow or take shape. And so a great evil of this world will be removed."

The Return of the King, "The Last Debate"