There’s no specific time frame that a truss rod should be adjusted as such. The idea of the truss rod is to counter-act the stresses and conditions the neck is faced with. The major factors in adjusting your truss rod include climate and string gauge. Using different gauges of string on your guitar applies different amounts of tension on the neck. This can cause the neck to have an up bow or alternatively, a back bow. The idea is to adjust the truss rod in small increments so that the neck is straight (or has a small amount of relief) when the guitar is tuned to pitch. If the strings are pulling the neck up into an up bow, the truss rod is what pushes the neck back down.
Even the same gauge of string can vary in tension across different manufacturers. For example, D’Addario 10-46 strings will apply more tension on the neck than Ernie Ball 10-46′s. If switching from one brand to another, the truss rod may need to be adjusted to compensate for the varied tension.
As far as climate being a factor, high humidity can cause the neck to warp without even having strings on it. Once you add the string tension into the equation, you can end up with an extremely bowed neck.
The best thing to do is to constantly keep an eye on your guitar neck and how much relief it has. You may find that it needs to constant adjustments, or it might need to only be adjusted once a year. Every guitar and every piece of timber is different.
About Mike Han
As a self taught guitar tech, Mike worked for many years as the on site guitar repairer for Steves Guitar Sales in Australia, before venturing off with his own successful repairs shop, Mikes Guitar Garage: Repairs and Setups. As one of the only guitar repair specialty shops in Sydney, Mikes Guitar Garage is always busy, yet still prides itself on fast turnaround time and attention to detail.
Mike is a also a working musician and has played in many various cover and tribute bands.