Once the tubes break in and stabilize they should hold bias in a stable manner and you should not have to check more often than once every six months.
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I check mine about every 2-3 months. They have been set @ 35 since I replaced my EL-34's with Gold Lion KT-77's last year, and I have not had to re-adjust the bias at all. Prior to changing the tubes, the original Svetlana EL-34's never wavered from their 35 settings in the three years of their faithful service. Happy Listening.