None or 0. Uncanny dodge cares about classes that grant uncanny dodge, not sneak attacks. So just because you have been granted the sneak attack, does not allow you to gain a completely different class feature.
Starting at 4th level, a rogue can react to danger before her senses would normally allow her to do so. She cannot be caught flat-footed, nor does she lose her Dex bonus to AC if the attacker is invisible. She still loses her Dexterity bonus to AC if immobilized. A rogue with this ability can still lose her Dexterity bonus to AC if an opponent successfully uses the feint action (see Combat) against her.
Improved Uncanny Dodge
A rogue of 8th level or higher can no longer be flanked.
This defense denies another rogue the ability to sneak attack the character by flanking her, unless the attacker has at least four more rogue levels than the target does.
If a character already has uncanny dodge (see above) from another class, the levels from the classes that grant uncanny dodge stack to determine the minimum rogue level required to flank the character.
So the wording is a bit strange, but take a barbarian, that class grants uncanny dodge, do even if that character never takes a level of rogue, they still benefit from their full levels for the purposes of uncanny dodge. The examples all use rogue in the description which is poor.