Доколе подойдет по смыслу. How long is not an exact translation, מתי shows when in time and is used at the beginning of the sentence when there is no certainty about the time factor and you have to ask a question. 'How long' shows a duration and does not specify a question about a time when the action would END. עד is definitelly referring to the fact that something will not continue forever and will end at a certain barrier represented by space or time or an object. עד הסוף, till the end, עד החנות till the shop, עד הקיץ till the summer, עד המקום כזה till this place, עד החושך untill the dark, etc.