This work investigates the long-time asymptotic behaviour of a diffusing passive scalar advected by fluid flow in a straight channel with a periodically varying cross-section. The goal is to derive an asymptotic expansion for the scalar field and estimate the time scale over which this expansion remains valid, thereby generalising Taylor dispersion theory to periodically modulated channels. By reformulating the eigenvalue problem for the advection–diffusion operator on a unit cell using a Floquet–Bloch-type eigenfunction expansion, we extend the classical Fourier integral of the flat channel problem to a periodic setting, yielding an integral representation of the scalar field. This representation reveals a slow manifold that governs the algebraically decaying dynamics, while the difference between the scalar field and the slow manifold decays exponentially in time. Building on this, we derive a long-time asymptotic expansion of the scalar field. We show that the validity time scale of the expansion is determined by the real part of the eigenvalues of a modified advection–diffusion operator, which depends solely on the flow and geometry within a single unit cell. This framework offers a rigorous and systematic method for estimating mixing time scales in channels with complex geometries. We show that non-flat channel boundaries tend to increase the time scale, while transverse velocity components tend to decrease it. The approach developed here is broadly applicable and can be extended to derive long-time asymptotics for other systems with periodic coefficients or periodic microstructures.