The best advice @Carlo Palomino is to "test"...Do a little research here - there were some great responses above...and then decide which Strategy seems the best one...and then test...If you want to improve your test results - Split test...Divide List and try 2 different intervals to see which one produces the most number of deals...In real estate its all about "Location, Location, Location" in Direct Response Marketing...It's "Test, Test, Test"...What you will find real quick when asking around is there is No "Magic Number"...

I'll not bore you with the details, but I will say that the tests I'm running are statistically significant; +-5% interval with 99% confidence.I hate to harp on the math, but it is quite important to this conversation. 

The 21 year interval mentioned is the time period for "adverse possession" in Pennsylvania; failure to serve notice would result in this situation.

Prior to my tenants moving in, I give them a welcome letter which also has the following information:- Copies of their lease agreement, and any other addendums they signed- The names and phone numbers to call to start utility service- Information on watering days (if applicable, because some of my properties are located in areas that can only be watered on certain days/times) and trash pickup days- Information on gardening service (if provided/applicable, I tell them who will be coming and on what days)- Location of furnace filters, size of replacements, and recommended changing intervals- Nearest schools- Information on renters insurance (I don't require it, but I give them info on it in case they choose to get it)- Location of the smoke detectors and carbon monoxide detectors, along with recommended changing intervals for the batteries- Any other unique information about the specific property/neighborhood itself that I feel is important for them to knowI also provide them USPS address change cards for their convenience since I figure they'll have a lot to be worrying about during their move.As far as the water heaters go, I don't really want them messing with it after I set it so I don't provide any specific information on it.

My adverising will be kinda "guerilla tactic to attract the most attention by use of a mobile Bill Board towed on the back of a small boat trailer which I can park for short hourly intervals a few times a week at various street corners in the geographic area of the rental.

When it is pumped, the pumping contractor can tell you if you might be able to lengthen the interval or if you should shorten it.  

1.)The bank will continue to lower the price at specific intervals until it sells.

Altering history was permitted in the past, but that led to confusing threads due to changes made after replies had been posted; so, altering history is only allowed for a brief interval, after which you can use the update feature to clarify your original intentions.

Mail in 6 week intervals so you'll hit the same 500 people 4 times in a 6 month spread.

You can get stuck with a property you are upside down on.So, it's good practice to run CMA's on your properties at some regular interval.