It will probably last till the weekend and will get too sour after that. The supermarket stuff lasts for weeks - but it is only because it is full of preservatives. 
To make home-made yogurt you only need milk (full fat, the fattier the milk, the thicker the yogurt) and yogurt. I'm not quite sure you can do this with UHT milk, but it is fairly easy finding a baba on a Sofia street to sell you real milk in a plastic bottle. :) You heat the milk to body temperature (that's if you put your finger in it you can't actually feel if it is either warm or cold), don't let it boil, than pour into a clean jar, put one or two spoonfuls of yogurt (best from the home-made stuff you have, the bacteria looks like it has done a great job), don't stir. Here comes the tricky part, you need to keep the jar with a lid in a warm place, not hot, as too hot and bacteria will die, too cold and bacteria will not be able to do its job. My nan usually wraps the jars in an old scarf and leaves them close to the печка (one of those in village houses that burns wood), for a lack of печка, my mum leaves them in the oven at the lowest setting for a while and than wraps them in an old sweater or baby blanket. :)
To make life easier, though Tefal has started selling a yogurt making machine! Def Bulgaria will be a good market for it! :) Good luck!