the question "when" is a general question asking about time. Normally, this question is answered by: a day of the week (on monday, on friday, etc) a month (in December, in March) a year (in 2010, in 2011, in 2012) or "yesterday, "later", "last night", "tonight" The question "what time" is more specific, normally you will expect an answer at what time of the day or as specific as "at seven in the morning, at nine in the evening or at 5:15 in the afternoon."

"what time" (几点？) is more specific. If I asked: "What time is he coming back?" (他几点回来？) then I'd expect an answer with a time, i.e. "He's coming back at 3:30 pm" (他下午三点半回来) "When" is more general. Let's say my friend wasn't coming back the same day, then I'd ask:"when is he coming back?" (他什么时候回来?) Then I'd might get an answer like:"He's coming back on December 3rd." (他十二月三号回来)