I like many have mentioned you tend to reap more benefits with a long term hold strategy and once you pay off your debt you get even more rewarded by reaping the full cash flow potential.
I tend to try and buy properties that I can maximize cash flow from the start even while servicing debt. I have also struggled about when to sell as I am a buy and hold and generally do not sell but as one gets to retirement 65+ one can either continue to manage the properties, hire management to ease your burden and let you enjoy life without the hassles of land lording, OR you can begin a sell off strategy where you can stagger your sales by selling 1 every other year or spread this out based on your need for cash and cash flow.
In the end it is a personal decision based on your own personal needs. Frankly, working a full time and managing 15 properties can be very stressful at times BUT if I didn't work a day job I think it would be very leisure to put around in my pickup truck and manage my properties. The biggest issue for me now is finding time while forking a day job in NYC with a hour commute each way.
OTOH, if by the time your are 65+ you may be just totally fed up with this business and want to cash out and hopefully by this time you have built up a good amount of equity in your investments if you bought right and managed right you should be in a great position to cash out and find truly passive investments if that's what you desire
It's also hard to predict how one will feel 15 20 years into the future. Things change and events happen also. Long as one has options which is the beauty of Real estate IE. Sell, refinance, owner finance, contract for deed, lots of options
I'm still in the accumulating stages so it's hard to think about selling :)
regards,
Chris