I have heard the oft quoted rule of having a positive cashflow of $100 per door on any RE investment, but I never liked this rule because anybody can selectively omit expenses to show a positive cashflow of $100 per door.
Oftentimes Sellers use far less than 50% of revenue for expenses to show "positive cashflow" so I was never really comfortable with this rule so I never use it. Entrepreneurs who are selling their business also gloss over expenses so it's not just confined to RE. A prospective buyer may have to dig awhile through 5 years of statements to find it (if they find it at all).
Sometimes in older buildings with deferred maintenance expenses can eat up far more than 50% of revenue for a significant period of time until everything is fixed up. Anything less than 50% of revenue for expenses makes no sense - for me.
With regards to how much David Lindahl brings in per month. I have no idea. With 7000 doors he's probably running some kind of giant JV or LP with probably hundreds (or thousands) of (limited/money) partners so his personal take is likely known to himself, his accountant, and the IRS.
However, assuming that he brings in $100 per door of profit per month, that means that his business is bringing in $700,000 per month in profit. Again, keep in mind that he only 'owns' a fraction of this depending on what is outlined in the JV/LP subscription agreements. This is a big (and likely incorrect) assumption as he has been doing this for awhile and many of his RE are in different stages of mortgage pay down, maintenance requirements, etc.