How long should I wait before I tell my employer that I am pregnant?

This is an unplanned pregnancy and I have been working for my boss since March, I am a marketing assistant and I really enjoy my work, it is only 2 days a week and I get paid **** all but am doing it for experience. Since doing this I have decided that I have no intention of working a 5 day week until my children have grown up (I have 2 kids, 5 and 2) as it takes me 3 hours to get to work and back each day and its a very long day for the kids having to be at daycare / school for 12 hours! Anyway I intend to stay at my job at very least til the end of the year, maybe all the way up until I have my baby.. when should I tell them?