Although a specific date is never revealed, it couldn't have been very long before the show started:
- Jerry tells his parents for the first time about the breakup in "The Stakeout," (Season 1, Episode 2) - the same episode in which the character of Elaine is introduced.* Jerry and his parents are close, and they talk fairly often.
- In the same episode, Elaine mentions that she's never seen him hit on someone else before. They obviously spend a lot of time together, and Jerry dates a lot of girls throughout the show, so it would be unlikely that too much time would have passed before Elaine saw Jerry chatting up another gal.
*There was no Elaine character in the pilot; she was introduced after the show was picked up by NBC. Apparently "Add a girl" was the only note the network gave the creators.More questions on Seinfeld (TV Series):