How long was Saber waiting for Shirou after she died?
Avalon is


  the domain of fairies... the unreachable utopia that Arthur dreamed of and was said to have gone to after her death.


In the Fate Route, after Saber vanished and returned to her own time, we see a shot of Arturia and Bedivere. It's assumed that from Arturia's line


  I think I will sleep much longer this time...


that this occurs after Bedivere third and final attempt to return Excalibur to the Lady of the Lake, as in the previous two attempts, when he was ordered to return the sword, he lied about returning it, knowing that by doing so Arturia would have died. So Arturia died and went to Avalon. Given it's a utopia I don't think Saber would be suffering.

Now Shiro is shown at the end of -Last Episode- to be hugging Arturia and he wa wearing the shroud Archer wore (though now in a different style), so they did meet up. This is why -Last Episode- is seen as the True End of the Fate Route, because Shiro and Arturia are finally together in the end. However, because Avalon transcends all magic, including the 5 Magics, of which The Fifth's domain is suspected to be time travel (but Touko says The Second governs time travel, so I am assuming that Avalon transcends time, or at least out conception of time), so despite the fact that Arturia died long in the past before Shiro was even born, we can't be totally sure that she waited the same equivalent amount of time for Shirou to even be born.

Shiro may have died to before being reunited, or he may have extended his life as some Magi do (without becoming a Dead Apostle). There's a series of H Dojinshin (fan made) which would assume that little time has passed since the time Shiro went to bring Arturia back. (One set has him and Arturia have sex in Avalon when they reunited, while in the next set, they are back in Fuyuki living together with Rin and Sakura visiting).

To my knowledge, it's not indicated how Shiro got to Avalon since we only know the conditions of the Miracle Shiro and Arturia achieved. I used eternity as a means to indicate an immeasurable amount of time, since given Shiro's Magecraft abilities, had he developed something to allow him to reach Avalon, it would have taken him a hell lot of time. After all, the ultimate ambition of every Magi is to reach the root, which can take generations to complete, since it took that long for the Tohsaka's to get close to properly using the Second Magic.