The in-game time that takes the random event to re-appear will vary for each of the encounters:
Characters for heists (Paleto, The Big One).
Example: Patrick McReary and Taliana Martinez
They are usually one time only, because the choice will change the available heisters during the course of the game history.
Characters for activities (Golf, Tenis).
Example: Castro Lagano
In case of a failure (the NPC died or got scared) he will re-appear during the next week.
Characters that give you stock (BAWSAQ & LCN).
Example: Gray Nicholson and Timm
They will reappear, but the time will vary according to the missions played and the distance between the encounter and the player. Most of the time is 72 hours for a failure, 7 days for the rest of the characters and between 1-4 weeks for the same character.
Note that sleeping will not make the random events re-appear faster.