Ask HN: Time between founder stock and priced round – tax implication?

How long should a company wait between incorporating (issuing founders stock) & taking in a priced equity round from investors? I recently incorporated and issued myself founders stock at a par value of $0.0001/share. I now have investors who want to make an equity investment of $500,000 for 25% of the company. I was told that I need to wait a period of time between from when I issued the founders stock and when I take in a priced equity round, so that I do not have to pay taxes on the founders shares that I received. How long does the IRS need that time period to be? 2 days? 2 weeks? 2 months? My investors may be able to wait, but I also recognize that the market may dramatically change in a short period of time, since I am in the tech industry. I've been told that it's better to take a check immediately, and I would image Ycombinator startups commonly have to incorporate and then immediately take in their funding check.