For a frenchie with clear signs, the practical answer is young: commonly between one and two years old, once the airway can be properly assessed but before years of obstructed breathing stretch and damage the soft tissues. Many airway focused vets assess at desexing age and operate on the clearly affected dogs then.
The biology behind the advice: an obstructed airway damages itself. The suction of every effortful breath progressively pulls the soft palate, saccules and larynx into worse positions, and late stage laryngeal collapse, the thing that caps what surgery can achieve, is the end point of that spiral. Operating early interrupts it; waiting until a heat wave crisis at age six means operating on a more damaged airway with a lower ceiling.
Older frenchies still benefit, so no dog is too late to assess. But if you are weighing up when with a young snorer, the honest answer is before next summer, not after it.
Send what your vet said and quoted. A veterinary surgeon reads it and comes back to you with what they can do and what it would cost, before you pay anything. A written second opinion is from $250.
Send the case, hear back from a surgeon with what they can do and what it costs, and only pay if you go ahead.