It is simple but not easy.
Let’s assume you want to reduce risk of capital investment and also risk of operations so we’ll rule out physical products.
That leaves us with digital products and services.
You need a business that you can sell to 20,000 people at $1 each, to 1,000 people at $20 each, 20 people at $1,000 each or 1 company at $20,000.
If you are going for the cheap value you have to mass market it somehow. Either by generating massive quality content or spending money in ads. 6 months is enough to generate leads to provide that A big enough following if you put effort into it.
If you want to go to the top bracket you will probably be in the B2B market. Since 20k is not pocket change you’ll most likely be playing the procurement game and each sale on this game can take anywhere from 3 to 12 months.
Pick your poison and good luck!