Several lines of dialogue in the first few episodes of Dragon Ball indicate that the balls have not been activated in several years, such as Roshi being unaware of their power, one of the balls being in a family for "generations". So approximately how long before Dragon Ball were the balls last reunited?
Well, there is no concrete information about when and how the dragonballs were used before the story in the manga began. The only info i know we got is from the first chapters, where Bulma explains to Goku what dragonballs are and why is she collecting them.
She also mentions then that the last person who used the dragonballs wished to become a king and after that the dragonballs disappeared for few centuries.
Bulma found a dragonball in her basement and since nobody knew what it was, she started to research it and stumbled on a story about them. That is basically how the story started( all this is referenced in the first chapter of DragonBall).