As Rita already said I was going to say it depends a lot on the variety you're growing, too. Some early varieties will set fruit quite quickly, as soon as the night temperatures stay above 55 or so, but then they might stop when the temperatures get really high as well. If it gets hot really early, they'll stop setting fruit sooner.
Some of the more heat-tolerant types will keep setting fruit in higher temperatures. My daughter in Utah is trying a new one (to us) called Sioux. It's actually an heirloom variety that boasts a "reliably large harvest even in hot weather" so we'll see how it likes the month or so of 100+ degrees they have in July.
http://www.tomatogrowers.com/M...
I have a hybrid I'm trying here called Solar Flare, that's supposed to last longer into our steamy summers. I think it will depend upon how resistant it is to the blights that seem to hit all my tomatoes in June.
