Is it "safe" to mine with your cpu on a laptop for a long time?
Unless it overheats there's no problem with nonstop mining. I'd suggest installing something like CoreTemp to view internal temperatures. You should adjust its number of cores used until all of the temperatures can be considered low enough. How low? 70°C should be fine. 100°C isn't very nice I think. There's no exact limit, but you can search online.

Overclocking? I can't comment if mining nonstop on an overclocked CPU is healthy for your computer. You might need to reduce the temperature by even more.