How long between spoke adjustments on a mountain bike? I have a mountain bike that's now about 18 months old. At the 12 month mark I took it for its first annual service. Since then there's been a problem with the front wheel coming out of alignment.

I've had the wheel re-trued a couple of times since the service now, it seems to need doing about once every 2 months. Obviously this regular attention is not a normal requirement for a wheel.

What is the normal period of time that you'd expect to pass between wheel alignment/spoke adjustment work? I'm riding the bike probably 20 miles a week on average on the road, with occasional weekend trips off-road. The wheel's gone out of true at least once without an off-road trip since the previous truing.
A well built wheel should go years without needed truing. If you are truing the wheel every 2 months something is wrong with the build. 

There's no way to know via the internet, but my guess is that it's one of two things. 


The rim is bent slightly and requires significantly uneven tension in the spokes to get 
the rim true. 
There isn't enough tension in the wheel overall to keep the wheel true. 


The way to diagnose this is to get a spoke tensiometer and measure the tension in the spokes, since the wheel is slightly offset to compensate for the braking disk, one side will have slightly higher tension than the other, but the spokes on any given side should be roughly equal. The rim should have a maximum spoke tension rating and the closer you 
can get the wheel to that the stronger and more stable the build will be. Most rims are rated to at least 100kg of tension.

A good wheelbuilder will have the spoke tension meter in his toolkit. They are relatively expensive and require some practice to get accurate readings.