Galactic Novae is at present well established class of gamma-ray sources. We wonder for how long the mechanism of acceleration of electrons operates in shells of Novae. In order to put constraints on the time scale of the electron acceleration, we consider a specific model for the injection and propagation of electrons within the shell of the recurrent Nova RS Ophiuchi. We calculate the equilibrium spectra of electrons within the Nova shell and the gamma-ray fluxes produced by these electrons in the comptonization of the soft radiation from the Red Giant within the Nova binary system and also radiation from the Nova photosphere. We investigate two component, time dependent model in which a spherically ejected Nova shell propagates freely in the polar region of the Nova binary system. But, the shell is significantly decelerated in the dense equatorial region of the binary system. We discuss the conditions for which electrons can produce gamma-rays which might be detectable by the present and/or future gamma-ray observatories. It is concluded that freely expending shells of Novae in the optimal case (strongly magnetised shell and efficiency of acceleration of electrons of the order of 10%) can produce TeV gamma-rays within the sensitivity of the Cherenkov Telescpe Array only within 1-2 years after explosion. On the other hand, decelerated shells of Novae have a chance to be detected during the whole recurrence period of RS Ophiuchi, i.e. ~15 years.