Hi there! First of all, beautiful beasts!
The mating of the cobras, for what I could observe depends really on the species. Some of them I keep (like N.siamensis) mated and made eggs after two and a half years. Some of the others, like N.melanoleuca, can wait a few years more. I'd say after three years, you can get them together, but don't be surprises if there's nothing soon... It could least for a while...!
Cheers
LucasC.