I think it would take more than a year or two unless they were given technical assistance by a current nuclear weapons state.
It's not particularly hard to make a nuclear explosion, but it is quite tricky to make a nuclear weapon.
The study they did a few deades ago called the 'Nth country problem' is somewhat illuminating here.
Remember that they do not need a weapon light enough to fit on a missile, the ROKAF and JASDF could easily get a combat aircraft far enough into North Korean airspace to deploy an iron bomb if they had to. It might cost them but they could do it.
Yes exactly! When you're trying to deter your next door neighbour then all you really need is a weapon which can be delivered by aircraft and such a weapon can be incredibly crude. Think Fat Man/Little Boy 2.0. And such a device is remarkably simple to create.
Of course in the long run you want a miniaturised warhead probably with variable yield suitable for mounting on top of a missile (and to shrink down the weapons for dropping from aircraft as well). Such devices, without outside help from someone who has done it before, take considerably longer.
But a simple device with no frills? Child's play at the end of the day. The only hard part is getting the plutonium or enriched uranium and I believe ROK has those resources within its existing nuclear industry.
I'm guessing people mooting Japan/JASDF dropping nukes on people are obviously slightly blind to two small historical events which means in Japan its practically enshrined in law to never develop, have or use nuclear weapons.
I'm not sure anyone is suggesting that Japan is anywhere near creating, let alone using, a nuclear weapon but that certainly doesn't change the fact that if Japan wanted such a weapon it would be easy for them to create it.
Do I foresee it happening at any point near or even not so near future? No, not at all. The only way I could see such a thing happening is if there is a massive geopolitical change in the region. Something along the lines of the US going fully isolationist leaving Japan to face China (and Russia) alone might be sufficient however for them to seriously consider it (and the changes required in law/constitution).