Answer: It is still very early in the season to look for ripe tomatoes. Depending on the variety you planted, 75 days or longer would not be unusual.
If the fruits are rotting on the bottom, that is blossom end rot. It is caused by uneven soil moisture levels. If the are splitting open or cracking, that is also due to uneven soil moisture.
Your goal in watering is to supplement rain to keep the soil evenly moist like a wrung out sponge, not sopping wet and not dried out. To know if you need to water, dig into the soil with your finger. If it is still damp, do not water yet.
When you do water, apply it to the soil surface and water thoroughly and slowly so it soaks down to the deeper roots. After watering, wait a few hours and then dig down to see how far the water soaked in; it can be surprising.
There is no set schedule for watering, it depends on your soil type and on the weather. Using an organic mulch several inches thick over the root area will help reduce watering needs as well as feed the soil gradually as it breaks down over time.
Here is a quick summary of a few of the more common problems that can affect tomatoes on the vine: (you may need to cut and paste the complete url into your browser to make it work correctly.
Too, when temperatures are high, tomatoes will ripen without turning completely red. The break point on temperature is about 95 degrees. Some varieties also do not turn a deep red when ripe -- color depends on the variety you are growing.
I hope this helps you trouble shoot.
Q&A Library Searching Tips