Really depends on the amp. I had a Music Reference RM9 that was easy to bias but needed adjustment every time I checked so I ended up checking it every week or so. OTOH, I have a Scott integrated that is a PITA to bias but once the tubes have settled in (about 200 hours), the bias is spot in each time I've check so now I'll check the bias once a year but have yet to make an adjustment (going on 5 years since i changed out the tubes). Another amp that's extremely stable is a Melody i2A3 which is relatively easy to check (requires a meter) and so far, hasn't needed adjustment (going on 2 years).
It comes down to the stability of the circuit as well as the quality and age of the tubes. Regardless of what volume level the instructions specify, don't play anything through the amp when making adjustments. I usually only have the amp on when biasing.