First of all, you can’t have a bad time in Paris for any length of time. Impossible. I’m having a great visit. However, that said, some times of year might be better to visit than others.
Based on my current visit, I would definitely not recommend August for an extended stay. If you are only coming for a week, and you just want to hit the museums, the public gardens, the Eiffel tower/monuments and the big department stores, then you will be fine. There are still many restaurants open. But if you want to know Paris more intimately, you will be disappointed. I’d say about 75% of the little boutiques are closed. The Folies Bergere, Maxim’s, the National Opera, etc. are closed. The art schools have been closed since June. The flea markets are about half full. Yesterday, I spent an hour researching where to go for a haircut here. Called both places. Closed until August 27th. On the up side, there are definitely fewer tourists and less traffic, so it’s a great time to do the museums. And the weather has been absolutely beautiful.
In July, other than the art schools and workshops (which was high on my list), most everything else is open. But the tourists are here in throngs, and many of them are really rude. The other issue is the heat and humidity. It was ungodly, swelteringly hot here almost the entire month of July. In Fahrenheit, high 80s to mid 90s. Really, really humid, too. And Paris has pretty much zero air conditioning. Even in some of the places where they supposedly have it like the L’Orangerie, you could barely tell. This meant my apartment on the 6th floor was stifling. I bought a fan (ventilator, as the French call it) for the apartment for $45, cheapest I could find. It helped a tiny bit, but it was brutally hot and hard to sleep. The restaurants are hot. If you’re getting around on the Metro in July, it is just oppressive. The cars are not air conditioned, and the air does not circulate. You get on, and about 5 minutes later, you’re a hot mess. The subway cars smell of body odor and spicy food gone bad. Also, everywhere you go here, you’re walking about 20 blocks at least and climbing tons of stairs if you’re using the subway. So in high heat, you’ll also be working up a sweat that way. If you come from a climate that is hot and humid, or you like the weather that way, then you’ll probably be fine. Personally, I am not a fan of extreme heat. I come from the Midwest (in America), which actually has a similar climate to Paris, though generally less humid. I wound up showering and changing my clothes three times a day. I don’t think I felt chic and up to Paris standards for more than 5 minutes the whole month. I think wearing sunscreen kept me from getting burned, but it also made me even hotter, because it doesn’t let your pores breathe. I was glad I opted for an apartment with a washing machine. I used it often.
My next visit to Paris will be either in May, June or September, which is what I’d recommend for most people. The French tell me that they did have an especially rainy June, but I don’t know if that’s typical. My last visit was for New Year’s, end of December beginning of January. It was around 45 degrees and rained most of the time. Most stores were also closed then, btw. It helps to do your homework here prior to visiting Paris. Happily, now I have done some for you!