It depends on the quality of the amp, the tubes and the stability of the incoming voltage.
My amp hardly ever needs rebiasing, even after a tube change. I check it every 6 months or so if I have nothing better to do, but it hasn't really changed in at least two years. It helps that it has robust and stable power supplies, uses current production power tubes that I always buy matched to the existing tubes and that my power conditioner provides stable voltage. All of these reduce tube drift, thus reducing the need for rebiasing or rebalancing.
That said, changing the bias does make a very audible difference. Bias adjustment provides a trade-off between harmonic extension and depth vs. tube life. But whatever number I settle on tends to stay put for many months.
YMMV of course, depending on all the above...
My amp hardly ever needs rebiasing, even after a tube change. I check it every 6 months or so if I have nothing better to do, but it hasn't really changed in at least two years. It helps that it has robust and stable power supplies, uses current production power tubes that I always buy matched to the existing tubes and that my power conditioner provides stable voltage. All of these reduce tube drift, thus reducing the need for rebiasing or rebalancing.
That said, changing the bias does make a very audible difference. Bias adjustment provides a trade-off between harmonic extension and depth vs. tube life. But whatever number I settle on tends to stay put for many months.
YMMV of course, depending on all the above...