The short answer: not July or August unless you have no choice. Summer is when St Tropez earns its reputation for gridlock, crowds, and prices that have lost all connection to reality. The D98A road from Sainte-Maxime is jammed from late June to mid-September; the ferry from Saint-Raphaël or Sainte-Maxime is the only sensible option.
May and June are the best months. The water is warm enough to swim, the flowers are out, and you can get a table at a restaurant without a reservation made three weeks in advance. The market still feels like a market rather than a tourist attraction.
September and October run a close second. The crowds thin after Labour Day, the sea stays warm through September, and the surrounding Maures hills take on autumn colour. Many restaurants stay open through October; some of the better ones close in November.
Winter is quiet almost to the point of emptiness — most businesses close between November and March. The village is genuinely peaceful and very beautiful, but there is little to do and fewer places to eat.
The rosé harvest in the surrounding Côtes de Provence vineyards runs through September — the wine-growing villages of Ramatuelle and Gassin are worth a visit at this time.