How passive it really is: Excellent – Once your ads are running, you could literally just sit back and watch the money roll in.  Again, you are spending money to make money here, so there is risk involved.  However, once the ads are set, the income can be very passive.  Keep in mind, you may need to monitor and possibly modify ads if they aren’t performing well.
When withdrawing money to live on, I don’t care how many stock shares I own or what the dividends are – I care about how much MONEY I’m able to safely withdraw from my total portfolio without running out before I die. A lot of academics have analyzed total market returns based on indices and done Monte Carlo simulations of portfolios with various asset allocations, and have come up with percentages that you can have reasonable statistical confidence of being safe.
What I Do: I’ve set up multiple investment accounts outside my main operations bank that deals with working capital e.g checking, paying bills. By transferring my money to a couple brokerage accounts and two other banks as soon as it hits my main bank I no longer have temptation to spend on frivolous things. As a result, I can wake up 10 years later and reap the rewards of compounding. My 401(k) is the best example where constant contributions over 18 years has grown to over $500,000 without any savings pain given it just became a part of life. Real estate is also a fantastic asset class for the long term. It’s fantastic to enjoy your home, pay down your mortgage each month, and end up with a paid off asset that has likely appreciated during your time of ownership. 
An obvious example is over exposure to bank stocks, which have been excellent investments for over a century.  Though a foundation of most portfolios, bank stocks do involve more risk at certain stages of the economic cycle than many realise.  Being less exposed to bank shares in the last few months could have preserved some capital.  So, a more diversified approach can help mitigate some of these risks.