works till

Alex+

"Do you know what time the restaurant works till?"

I asked this question to the receptionist in a hotel but he didn't understand me. Is this sentence not correct?

I'm not sure but maybe you can put it this way:

Do you know what time does the restaurant close at?

When does the reastaurant close? 

The most common ways to ask this question would be:

What time does the restaurant close?

When does the restuarant close?

How late is the restaurant open?

Some prepositions, especially those related to time, don't easily tolerate stranding.

You work till what time? becomes Till what time do you work? (not stranding the preposition at the end)

Or, with stranding:  What time do you work till?

till just doesn't work well when stranded at the end.   Nor does its "opposite" since.

What time have you been working since? is thus awkward, while Since what time have you been working? is not quite so awkward.

In any case, most of these are handled by native speakers through rephrasing:

What time do you get off work? (??What time do you work till?)

What time did you start working? (??What time have you been working since?)

CJ 

It's not incorrect, but it's imprecise.

If I'm working on a major project, the answer to the first question may be "three weeks should do it" and my answer to the second could be "probably when I'm about 65."

What time asks a more precise question than when.