Should we wait for the end of a symphony or a concerto before applauding? Yes indeed, if we want to avoid the condescending sshhh! of our neighbours at Salle Pleyel, Carnegie Hall or any other concert hall.
Yet... applauding not only between the movements of a symphony but even during its very performance (at the end of a sequence or when an effect calls for it) was a customary practice in the 18th century, just as it is today in jazz clubs. And the lack of this applause, so 'out of place' according to contemporary etiquette, was quite simply the sign of a patent failure, as much for the composer as for the performers. A real flop.
'In the middle of the first Allegro, there is immediately a passage that, I well knew, would please. All the listeners were filled with enthusiasm - there was great applause,' Mozart wrote to his father on 3 July 1778 regarding the first performance of his 'Paris' Symphony. Even more interesting is the next part of this letter in which, contrary to our generally accepted Judeo-Christian ideas about composers of 'serious' music (Mozart would have died laughing at this definition), Wolfgang provides us with a veritable custom-made recipe for charming the Paris audience and provoking its applause while (!) the orchestra is playing: 'Having heard that here, all the last Allegros begin like the first, with all the instruments together, I began piano with two solo violins for only eight bars - then immediately came a forte so that the listeners (as I expected) applauded.'
Be reassured: all these historical specifications, however accurate, in no way change the behavior that you are supposed to adopt in a concert hall. In the same way that your confessor would send you to see a shrink if you said that God had appeared in your sleep to inform you that henceforth you could have direct contact with Him without going through the Church, so will your neighbors at Carnegie Hall take you for a hick if you do what Mozart himself would have loved you to do, i.e. applaud in the middle of his 'Paris' Symphony.
As luck would have it, at eight hours' flight from Paris, there still exists a great Mozart lover with truly 18th-century tastes: in one of his youthful one-man shows, Woody Allen expressed his entire satisfaction with a standing ovation, albeit the one he received at the very beginning... of his wedding night.
But is there an orchestra in the whole world with morals so joyously Mozartian that you might applaud when you feel like it and even give a standing ovation at the very beginning? Yes, and it is to be found in London: The Orchestra of the Age of Enlightenment. Considered the orchestra of reference on period instruments, this British ensemble, which can be heard in the most prestigious venues with 'traditional codes', has also established the series The Night Shift where you can not only applaud but also talk, flirt, have a beer--in short, live--while listening to Bach or Mozart. If this were an event concept of a mediocre orchestra seeking notoriety, it would be suspect at very least. But at this level of musicological knowledge and artistic excellence and (more than anything) inspiration, it is truly worth a trip to London.