[Boromir88]

I went back and read Gandalf's story to Frodo regarding Gollum, and I think it's reasonable to say there are several reasons behind the length of the time frame, either that or Gandalf had no idea and was just making stuff up.

As CSteefel points out, I don't know if Gollum vacillated between Sauron's calling, and the Ring, but Gandalf certainly believed there was internal conflict and the Ring hadn't wholly ruined Gollum:

"Only too true, I fear," said Gandalf. "But there was something else in it, I think, which you don't see yet. Even Gollum was not wholly ruined...There was a little corner of his mind that was still his own, and light came through it, as through a chink in the dark: light out of the past. It was actually pleasant, I think to hear a kindly voice again, bringing up memories of wind, and trees, and sun on the grass, and such forgotten things."

Now Gandalf goes on to say that this only fueled Gollum's anger, and Gollum being "cured" was near impossible (though not beyond hope). But to go back to something alatar pointed out, was Gollum so strongly "addicted" to the Ring?

"...For it was long since he had worn it much: in the black darkness it was seldom needed. Certainly he had never "faded". He is thin and tough still. But the thing was eating up his mind, of course, and the torment become almost unbearable."
"...He was altogether wretched. He hated the dark, and he hated light more: he hated everything ahd the Ring most of all."

So, there certainly is the internal conflict, the calling of the Ring after Gollum loses it, and his attempts to resist, until it became unbearable and he left the mountains.

Also, I imagine Gollum's trek was very slow:

"Light, light of Sun and Moon, he still feard and hated, and he always will, I think; but he was cunning. He found he could hid from daylight and moonshine, and make his way swiftly and softly by dead of night with his pale cold eye...He found his way into Mirkwood, as one would expect."

Even though if he could move "swiftly" if he's only moving around the dead of night, to avoid both the sun and moon, he can't be spending too many hours "on the move." Plus, his physical condition, and while he did start gaining more strength as he was able to catch more food, he left the mountains "mortally hungry."

One more possibly interesting thing to point out about Gollum's character, is his curious nature; or need to "uncover" secrets:

"All the "great secrets" under the mountains had turned out to be just empty night: there was nothing more to find out, nothing worth doing, only nasty furtive eating and resentful remembering."

So, alatar, maybe your random walk makes sense for Gollum's character? Or his need to search through all the niches and secret passages in the unchartered Mordor? The Dead Marshes? Thus explaining his vast knowledge of the area (his vast knowledge of any area he's spent time in...as Gandalf says he was able to avoid capture by the Elves while in Mirkwood) and giving Gollum something else to do besides thinking about getting the Ring.