This is what I've learned, and other folks I've talked with tend to agree . . .
When the plants are still young and small, water only needs to be flushed once every 7 - 10 days, but, when they get larger, and start consuming more nutrients (this also depends on the type of nutes being used, temp & lighting intensity), the more often you flush, the better.
Some have even suggested every 2 - 3 days. That may seem like a lot of work, and it can be, depending on how your hydro system is set up, but the idea is that you want the plants to be as "happy" as possible, and they do like fresh, clean water with just the right nute mixture. You can tell pretty soon, within hours or a day at most, if they are thriving well, or are just sort of getting by.
I understand this also translates into providing nutrients more often, which may seem like a bit of an added expense, but here's the crucial detail that might be the most relevant - it's better to keep giving the plants fresh new water, with a fresh, but carefully measured nute mix (DON'T overfeed), as often as possible, rather than just giving them a big blast of nutes and fresh water less often.
My thought would be, if I'm going to go through with all this initial work, and money put into this project, then I don't mind being a little bit on the fanatical side when it comes to putting in the extra effort, to make these plants as happy as they can be.