Slow-roll e-fold count
= Slow-roll e-fold count
The number of e-folds between field values $\phi_i$ and $\phi_f$ is
$$
N\simeq\frac1{\sqrt2M_{\rm Pl}}
\int_{\phi_f}^{\phi_i}\frac{d\phi}{\sqrt{\epsilon_V(\phi)}}
$$
when the field rolls monotonically down the potential.