  Magnetic chemically peculiar stars of the main sequence can present
rotational periods as long as many decades. Here we report the results of an
observational campaign started in 2001 aimed at establishing these very long
periods from the variability of the integrated magnetic field modulus, the
so-called surface magnetic field $B_s$, as measured from the Zeeman splitting
of the Fe{\sc ii}\,6149.258\,{\AA} spectral line. Thirty-six stars have been
monitored with various high-resolution spectrographs at different telescopes,
totalling 412 newly collected spectra. To improve the phase coverage, we have
also exploited all public archives containing high-resolution spectra, many not
yet published. On the basis of these new $B_s$ variability curves, we 1)
confirm or revisit the periods of 24 stars, 2) extend the lower limits to the
periods of HD\,55719 ($P > 38$\,yr), HD\,165474 ($P > 27$\,yr), HD\,177765 ($P
> 37$\,yr), 3) establish for the first time the periods of HD\,29578 ($P =
10.95$\,yr), HD\,47103 ($P = 17.683$\,d), HD\,150562 ($P = 5.7$\,yr),
HD\,216018 ($P = 34.044$\,d), and 4) set lower limits to the periods of
HD\,75445 ($P >> 14$\,yr), HD\,110066 ($P >> 29$\,yr), HD\,116114 ($P >
48$\,yr), and HD\,137949 ($P > 27$\,yr). As to $\gamma$\,Equ, whose period must
exceed 90 years, we point out a clear decrease in the field modulus, the
maximum of which coincides within the uncertainties with the minimum of the
variation in the integrated longitudinal field.
