How much persistence does it take to succeed in a business venture?

As much as it takes.

The more brilliant you are, the more resources you have to get it off the ground, the luckier you are, the more you are in the right place at the right time with the right product or service, the faster it'll be.

But it always takes as long and as much as it takes.

If you have any other thought in mind, don't bother starting a business.