I recommend that Disto Xs (the A3 version) be calibrated monthly. I haven't had the opportunity to test at weekly intervals, but most of the time we calibrate a day or two before use and haven't normally used a Disto X for more than a week after a calibration anyway.
The general rule of thumb is that if the FS/BS don't agree, check the reader for interference first and if FS/BS still don't match (within your standard), assume the calibration is bad for one reason or another (old calibration, instrument has been dropped, been too close to large magnetic field, batteries dying, or ???) and switch to your Suuntos. NEVER use a Disto X without taking backsights - you will never know if something goes bad with your calibration. Mostly I have found that when an instrument starts having trouble with backsights agreeing (and the cal is new and was working fine) the batteries are running low. Although the magnetic fields of lithium batteries change less than other types, they change rather rapidly when they are running low.
This is what I have seen in previous testing. There is a slow spreading of the azimuth data. The elevation data doesn't change much (as you might expect). The graphs are Disto X compared to Theodolite data, error in Az and El plotted. If you can't read the scale, it is in 0.5 degree increments (plot from +2 degrees to -2 degrees.