Back to the seedling tomato plants for spring planting in the garden, the time that it takes for the seed to germinate are going to be largely determined by the average temperature of the potting soil or the soil that you planted them in. So the warmer that you can keep them in the sunlight is the best thing to warm it with, the better. Ordinarily the seeds will take about 10 days maybe as much as 2 weeks to come up again depending on the temperature. During the meantime, you will want to keep the soil moist by don't soak them everyday. You will be able to tell by looking at the top of the soil that it is beginning to get dry. When the top of the soil begins to get a little bit dry, the underside is still moist enough to support the life of the tomato seed which is putting down a root first before it ever puts anything coming up. It is going to put down a root first. So you can be sure when you see those little green sprouts coming up in your cells, that you've got a tomato plant that is already established root and just needs to be kept with a certain amount of moisture, not soaked but enough to keep the plant going. Generally speaking you can tell that by looking.