Twice a year.  They really don't get that dirty.  Buy a 3 pack at Costco or Sams Club and you will always have an extra one around and cheaper too. 

I never change them, I have a guy named Nick who runs around town changing them.

When my thermostat reminds me.

Air filters, what's that? 

Nicholas Crane I always used to forget to change them. I purchased a whole-house electronic air filter. I have been cleaning it about every six months.

I change the filter every month. I have heard that the correct air flow is very important for the efficiency of the system. I have been told by HVAC service people that the thick filters are too restrictive and reduce air flow. 

Every month on the 15th.

Thank you for the reminder. 

We clean them every other month.

Every other month and I buy the 3  pack so there's always at least one available. It's important Nicholas!

Good morning Nick. A minimum of every thirty days; never over 60.

Good morning Nick. In our home and office, at least once a year, in my vehicles, at least a couple of times a year.

Make it a great week!

Twice a year but should probably be every month since I live in one of the top 5 dirties air cities.

monthly in summer and every three months the rest of the year.  We heat with a wood burning stove so our furnace runs very little.

I only use wood heat. There's no furnace in my house so I don't have any filters to change. I'm not joking. 

probably every 6 months.

It depends on how dirty the air is. I have an electrostatic air filter and it gets really dusty so I would say every 6 weeks.

Depending on the place (rental 2 to 4 months.)