You should be aiming to earn between 40-50,000 baht a month if you are working in the capital. This will give you a fairly comfortable standard of living. There are of course people who survive on a lot less in Bangkok (25-30K a month) but I really don't know how they do it. If you are earning 25K a month in the capital, then you are seriously going without. In rural areas however, teachers say you can live like a king on 25K. It totally depends on the individual and their spending habits of course.
Take a look at our 'cost of living' feature, where numerous teachers describe how much they earn and what they spend it on. You'll find it interesting to compare lifestyles.