You should start getting ripe fruit anywhere from 2 to 4 months after planting out in the garden. Different varieties have different DTM (days to maturity), so some will ripen 70 days after transplanting and some will ripen late, 100 days or more. Usually when you purchase seed, the description will tell you the DTM.
You can cut or twist the fruit off when it is fully colored. Some heirloom varieties ripen green or have green shoulders when ripe; Google the varieties you are growing to see what they look like when ripe.
From this point forward, it's mostly watering and picking fruit till the season ends.
Here are a couple useful links to help with any problems the tomatoes may have:Diagnose tomato disorders/diseasesTomato pests