Mounting CB radio overhead, How long can the screws be?

[abig84]

I mounted my dash cam mount up front all I did was carefully drill through the first layer of metal then just pushed the drill bit through til it touched the outer roof and marked the drill bit and found screws that were appropriate length and installed them with a hand screwdriver
I did it like 4 years ago so don't really remember the length of them.
I just figured it's best not to go ramming self tappers in there