Is it safe to record long videos with a DSLR?

I have a nikon d 3100, i know you can record 10 minutes at a time, but you can easily press record again

but i have heard about "the sensor over heating"

so, is it ok to shoot long videos, 30 to 60 minutes on this camera?

It's safe. If the camera allows it, then you have nothing to worry about.

Yes it is safe. Once sensor gets over heated, first it would give warning sign on the screen and then it would shut down.