  In the present paper we provide a formula that allows to compute the number
of stable digits of any integer tetration base $a\in\mathbb{N}_0$. The number
of stable digits, at the given height of the power tower, indicates how many of
the last digits of the (generic) tetration are frozen. Our formula is exact for
every tetration base which is not coprime to $10$, although a maximum gap equal
to $V(a)+1$ digits (where $V(a)$ denotes the constant congruence speed of $a$)
can occur, in the worst-case scenario, between the upper and lower bound. In
addition, for every $a>1$ which is not a multiple of $10$, we show that $V(a)$
corresponds to the $2$-adic or $5$-adic valuation of $a-1$ or $a+1$, or even to
the $5$-adic order of $a^{2}+1$, depending on the congruence class of $a$
modulo $20$.
