Andrew's responses are all perfect. Some additional ones are: What time exactly do you want to meet ? Any special time you want to meet ? What time would you like to meet ?

If someone were to say to me "Let's meet on Wednesday". If I was not busy, I would then simply ask "What time?". Or if I was busy that day I would ask "What times work for you?". I think other natural ways to ask include: "What time were you thinking?" "Is there a certain time that works for you?" "Do you have a time in mind?"