Could be a slow leak in the tire, a poor seating along the bead, a valve stem problem, or if you have aluminum wheels - a porousity problem.
General rule-of-thumb I've gone by is to check the pressure once per month - mainly to correct for pressure variances caused by average ambient temperature changes.
One trick is to fill a spray bottle with soapy water, then spritz the tire and look for bubbles. As slow as your leak is described, you may be chasing your tail - but might be worth a shot if the airings are too annoying. IMO, if it's only a couple of pounds every month or so - I'd just be sure to check pressures regularly.
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1989 F150 XLT standard-cab shortbed 4x4, 5.0L, AOD, 3.55 gears, manual hubs, 31x10.50 tires, 212K miles. Repainted Oct09.
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