Since when, I haven't seen you? 》 When was the last time I saw you? Since when, we haven't seen each other? 》 When was the last time we saw each other? Since when, we haven't done this? 》 When was the last time we did this? // How long has it been since we have (we've) done this? Since when, we haven't come around here? 》 When was the last time we came here? // How long has it been since we last came here?

It's not as common as you might think. You will surprise everyone if you say it for 2 reasons: 1. It's unexpected from a student of English 2. It's unexpected from an educated young lady as yourself You could use it with close friends as joke when you are pretending to be upset with them, as if saying you will beat them up, but please don't make this phrase part of your everyday conversations! lol UrbanDictionary.com is a great resource for looking up the meaning of slang. It's a good idea to look it up and understand the meaning before adopting foreign slang.