How long can I marinade/brine a tenderloin?
There is a difference between brining and marinating.  Brines are salt based solutions that work to make meat jucier.  It works by osmosis.  First water leaves the meat, then returns with the salt from the brine.  Because the meat cells now contain more salt, water remains during cooking.  Brining means juicy meat, but it is also salted.  This impacts flavor and texture.  In my experience some folks like this, other don't.  Marinades can add flavor.  Some marinades break down proteins and impact texture (depending on ingredients).  Marinades don't add moisture.  Marinades (as opposed to brines) only impact the surface (or slightly below...maybe 1/4 inch).

Proteins can be brined for hours to days depending on the process and desired impact.  

Marinades are usually recommended for 1 - 12 hours.

So, to fully answer your question you would need to specify the impact you are looking for, but in general, given the short cooking time, pork tenderloin is not brined.  You probably want to marinate, and I would suggest a maximum of overnight.  Because there is little penetration, longer is not better.