1-6 months, natural disasters or economic collapse
The U.S. Ohio-class submarine can carry 24 Trident II submarine-launched ballistic missiles. They can have up to a 7,000 mile range, depending on payload.Those subs only have a 500 mile shooting range. Russia has the posiden torpedo which is nuclear powered -so infinite range- and carries autonomous nuclear drones that can be deployed sitting off a countries coastline.
Each can re-enter the atmosphere at Mach 24, can divide into numerous individual nuclear payloads ranging from 100-475 kiloton nuclear warheads.
A single Ohio-class sub, launching its full payload, could throw up to 192 nuclear warheads between 4 and 7K miles away.
At any time, we have 12 of these subs in the water, with 2 in overhaul. Each with 20 missiles. That's a total world-wide nuclear arsenal of up to 1,090 warheads from our subs alone.