20 to 30 days Click to see full answer. Keeping this in view, how long does it take for a tomato to turn red? between 20 and 30 days Subsequently, question is, how do you turn green tomatoes red? One way how to turn green tomatoes red is to ripen mature green tomatoes in a well-ventilated area at room temperature, checking their progress every few days and discarding unsuitable or soft ones. The cooler the temperature, the longer the ripening process will take. Subsequently, one may also ask, why is it taking so long for my tomatoes to turn red? When temperatures exceed 85 to 90 F, the ripening process slows significantly or even stops. At these temperatures, lycopene and carotene, pigments responsible for giving the fruit their typical orange to red appearance cannot be produced. As a result, the fruit can stay in a mature green phase for quite some time.Why won’t my tomatoes turn red on the vine?Tomatoes won’t turn red if it’s too hot (above 85F) or too cold (below 50F). Also, as tomato plants mature through the summer, they can become huge and overgrown. When that happens, they tend to spend most of their energy on growing leaves and flowers, rather than ripening tomatoes.