IMO The best time to say something would be at the time and it's difficult to explain without the plate of food to demonstrate. As it's still quite recent, this may help and it's feasible they'll send a credit note and/or have a quiet word with the chef.
However I've eaten (or not if it's too bad) some poor meals and not said a word. All I can then do, is either give them another chance or vote with my feet. I recall two separate meals in Italy that they so patently didn't care about the food, that I can't believe they would have been interested in a discussion (two prime examples of avoiding crappy tourist places!).
I guess these are the options open to you, though if you do go back and are disappointed you can mention it to them in the clear knowledge that they either demonstrate a willingness to listen, understand and/or correct (in which case they keep your custom) or they're not bothered and you can scrub them off your list.
I'd be very interested to hear from those who've spent time on the other side of the hatch who post here.
Not easy - I hate complaining