This paper discusses the time evolution of a slow Reversed-Field Pinch during the sustainment phase, starting from an initial low-β configuration at the end of the setting-up phase which corresponds approximately to a Taylor Reversed-Field Bessel-function state. Calculations are made with a one-dimensional MHD (1DMHD) equilibrium and diffusion code. Because of local overheating and consequent rise of plasma pressure the central region of low shear becomes Suydam-unstable, and it is assumed that this results in local MHD turbulence, so flattening out the central density and temperature profiles and leading to a quasi-steady state which gradually evolves in time. The pinch then consists of two main zones, a central Suydam-unstable core I and an outer MHD-stable layer II which is responsible for confinement. An external zone III may exist near the wall but is not studied here. The configuration time τc depends on the time for which the trapped positive Bz flux ψ+ can persist against resistive diffusion. This in turn depends on the electron temperature in zone II and hence on the anomalous electron thermal conductivity that is assumed in the model. Some information can be obtained by normalizing the phenomenological transport coefficients used in the code to the measurements made on ZETA but this does not provide a very sensitive check. Further information can be obtained from empirical tokamak scaling laws. Then the performance of the proposed device RFX is predicted. A significant difference between ZETA and RFX is that in ZETA it was the disappearance of the negative flux ψ− that terminated the quiescent period, while in RFX it is the positive flux ψ+ that controls the configuration time. This leads to a substantial improvement in predicted performance.