My Totota Venza's tire pressure light keep coming on very 6 weeks after driving it for a year. Only have 10K miles after 1 1/2 years. I have taken it back to dealer three times and will be a fourth time now. They couldn't figure out what's wrong with my tires. They just turned off the lights, inflat them, and returned it to me. Is it normal to check for tire pressure every week/month? Is it a defect on my car?

No car defect - there is a normal amount of wear on your tires after 10K miles, and
one (or more) of the tires are starting to leak air just a little bit - which after 1.5 years and
10K miles is entirely expected.
What's happening is that the TPMS (Tire Pressure Management System) sensors (one in each tire)
are sending info to the car's computer telling it that there is low air in at least one tire.
To the TPMS, "low air" means a drop in air pressure of just a few percent (set by the
manufacturer). A drop in pressure in just one tire will light the dashboard light.
The spare (temporary) tire is not monitored by the TPMS, and typically carries a pressure
of 60 PSI.
According to http://www.safercar.gov (NHTSA), a normal tire will lose as much as 1.5 PSI
per month as air escapes the tire and rim naturally.
The 2009 Toyota Venza utilizes the "direct" TPMS system (as opposed to the indirect), which is
explained by the Wikipedia article at http://en.wikipedia.org/wiki/TPMS

If you or dealer suspect the TPMS sensor(s) in one or more tires, they can be purchased on the aftermarket from sites like www.rockauto.com, www.carpartsdiscount.com, et.al. After the new TPMS sensor(s) are installed,
you (or your garage),must re-teach the TPMS system its new sensors using the following procedure: (1) let the car sit stationary on its wheels (not lifted) for at least 20 minutes, then
test drive the car for at least 10 minutes at a continuous speed of 15 MPH or greater. During the
test drive, the TPMS dashboard light should go out.

For 2009 Toyota Venza recalls, visit
http://www.autorecalls.us/aut-03/recalls/2009/toyota/venza/index.html

It could be a problem with the sensors that check if the tire pressure is low. Have them checked out or even replaced to see if that fixes the problem.

Tpms will go come on if air pressure gose to high or low it is probly to high or to low look on side of door on driver side will give specs for air in tire pressurecheck best when tires are cold unless you use nitrogen it dosent expand when heated