Hello and welcome. In homebrewing longer is almost always better. It's not easy to do with your first one when you can't wait to try it. You are doing great following the instructions for the first time. That said many of us for a "normal" beer like the Dead Ringer would not use a secondary at all. You will see about that as you go on.
How long was the primary? Chances are it was done after that but the secondary will give it time to finish if it isn't fermented out and to clear some. I'll go out on a limb and say a couple weeks secondary should be plenty but more will not hurt.
The only way to know for sure is to get yourself a hydrometer and flask. You will see in the instructions the OG or original gravity is 1.064 The kit should have come out to pretty close to that. When the beer is done it will have a FG, final gravity. At the point you think it is finished you can take a gravity reading for a few days in a row. If it is the same, it is probably done. A typical FG reading is around 1.01x. If it's much higher than that ask us here about it and we will help.
To get your sample a wine thief works great https://www.northernbrewer.com/products/fermtech-wine-thief
Sanitize it like the rest of your stuff, pull the sample and drain it into the flask. After the reading drink or dump the sample. No reason to risk an infection to try to save the small amount.
I know that was a lot to take in so for now just keep and eye on it. At some point your air lock will stop or almost stop bubbling completely but bubble watching is not for sure like the hydrometer. Why the homebrew starter sets all do not include a hydrometer I do not understand.
Feel free to ask any questions. We have all been there.
EDIT: Some redundancy here because @Rburrelli must type faster than I do.