"what time" asks specifically for time e.g. hour, so in Turkish this is "saat kaçta". On the other hand "ne zaman" and "when" are broader questions
but it's complicated because 'ne zaman' does literally translate as 'what time', and 'saat kaç' as 'how many/much hours'. I guess 'what time' isn't accepted as a translation of 'ne zaman' because of cultural reasons more than because of the literal meanings of the words, i.e. because the Turkish concept of time is more separate from its measurement than the English concept of time.