For both of these objectives, the goal is to obtain 3 skyshards (unless another number is stated by the encounter).
For the survey objective, the delve is successful and automatically ends at the end of the round once the last skyshard is gained.
For the uncover objective, you end the delve by choosing to retreat. This means you can continue to additional rounds even after the last skyshard is gained, if you wish - though you can no longer explore.
As well, if you retreat early from an uncover delve, you still get XP equal to the number of skyshards gained, whereas in a survey delve you gain no XP for gained skyshards. Note though, that retreating before finding all of the skyshards is still considered an unsuccessful encounter, and other rewards are not gained.