This question usually refers to Alison DiLaurentis in Pretty Little Liars. For the TV show audience, her being alive is clearly confirmed partway through season 4.
Episode of confirmation
In Pretty Little Liars, it is explicitly confirmed that Alison is alive in season 4, episode 12, titled “Now You See Me, Now You Don’t,” when Carla Grunwald explains that she pulled Alison from the shallow grave and that Alison survived. The Liars then actually see Alison alive at the end of the following Halloween episode (season 4, episode 13 on many listings), which visually confirms to viewers that she is still living.
