When you say heated greenhouse, what temperature are you maintaining? and have you had grow lights on them?, tomato's need a min temp of 50 deg f, and a max of 80, and also a lot of light to develop properly, I would have thought it a bit early in the season to have plants that size.
Watering should be done when the soil feels a bit dry to the touch, but do not allow them to dry out, it's impossible to say exactly when to water as there are so many variables, you can't for example say water every third day, or every other day, you have to get to know by feeling the soil, it's not easy for some, but you will learn given time, Derek.

Thanks Derek, the toms I have were given to me by a friend from Tenerife, he brought them over in January, at that time they were about 6 inches tall. I have brought them into the conservatory where the heat is more controllable.