You could go either way. If you need to do a diacetyl rest, now is probably the time. Otherwise, it's up to you. For a lager, I would probably go to "secondary" (i.e., lagering) very shortly after the final gravity is reached, but others leave it for a while after that before lagering with great success.
So, if I were you, I would taste test for diacetyl. If there is even a hint of buttery or butterscotch flavors, smells, or mouthfeel, I'd raise it up to 65°F or so for 2-3 days, then bring it back to the fermentation temperature. If it is diacetyl-free, then I'd wait a couple days and check the SG again. I'd repeat that until the SG reading is stable, then lager. At that point, you can essentially wait as long as you like, though there's a bit of risk that you'll get some unwanted flavors from the yeast if you want a LONG time. (A lager is somewhat more susceptible to this than an ale.)
For my own first couple lagers, I did the D-rest for about 3 days after it had reached around 1.020, then I lowered the temperature slowly (around 5°F per day) until it reached lagering temp. I didn't bother checking the SG, I just trusted that 3 days at high temperature and another week at a slowly decreasing temperature would be enough for things to finish up on their own.
