If you were on forums early on, search yourself in ascending order. I just did it to find my old threads.
if you're playing on iPhone you can check your purchase history on your Apple acct and see when you downloaded the app (I think) prob same for google play ..but idk
You can check how long you've played by going on the forums through a computer. You can also check it by looking at the KaWmunity achievement and see how long ago it was earned (only possible on PC). As far as I know it's not possible to view this in the app version of the game.