The highest I've ever taken a character I've played from level 1 is level 5. I seem to have a knack for dying just before hitting level 6 (like, literally in the encounter that would get me the XP to level...).
The longest level run I've ever played in one continuous campaign (regardless of how many characters it took me), is probably... five? Huh. Depends on if you count the one campaign where after a TPK we started a new bunch of characters, on a completely different quest, but at the same level and in the same world. In that case, 9 or 10. (The same character ended up migrating over into a completely different campaign, by a different DM, and has reached level 12).
In the campaign I DM, the players have just hit level 8, and two of them are still playing the same characters they've had since the start. Everyone else has had to replace their character at least once, with the highest turnover being a player who is on his fourth character.