Any more than about 8c at 440 (~ 2Hz).
This has always confused me. If one does not tune to A440 exactly each time, don't you end up having to retune the treble and bass each time?
Having maintained pianos in many cases where I've returned to them on a regular basis over time, I don't think this is a good method any longer.
Initially, I thought it made sense: the soundboard is rising and falling, so shouldn't we put less stress on the piano by tuning the midpoint?
What I've found is that in most cases the strings lose tension in a pocket in the middle, while the more supported (less mobile) areas of the soundboard tend to remain stable. Every piano has a unique signature of how it loses tension - but always starting in the center. So, the treble (above C6 or so) and the bass (below C2 or so), tend to remain stable.
If the tension is not recharged in the center pocket, then the treble and bass will begin to lose tension gradually and soon the piano will require a more drastic pitch raise. Conversely, if the extremes of the piano are corrected to the center pitch, the need for a more drastic pitch raise will be created later.
Over periods of soundboard movement, the strings do gradually lose tension. The piano cannot be modeled as a sine, but more accurately as a sine following a downward curve (with respect to net tension over time).