This could be interpreted two ways: "Tell me at what time you finished the work (did it)." Or "Notify me when you have finished the work (done it)". The first type is asking for a specific time, the second type is asking to be notified after the work is finished. If we are asking for a specific time, it may be better to be more clear: "Tell me the time you did it at." or "Tell me the time at which you did it." But "Tell me when you did it." still makes sense in this context. If we are asking for a notification when the work is done, we need to use a different wording: "Tell me when you have it done." or "Tell me once you have it done." or "Tell me after you have done it." For this situation, "Tell me" is not often used. Most people will say "Notify me" or "Email me" or "Call me" and so on. So: "Call me when you have it done." or "Email me once you have it done." or "Notify me after you have done it."

The second sentence sounds a bit more awkward than the first, but both sentences are okay for casual conversation (although those two sentences might be used by people who are less educated and do sound just a bit off). A much better sentence structure is, "Tell me once you've done it." It is understood in that sentence that the person should let you know when they are done.

Tell me when you do it. Suggests that it will happen in the future. Tell me when you did it. Suggests that it has already happened.