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Homogeneous oscillations of the inflaton after inflation can be unstable tosmall spatial perturbations even without coupling to other fields. We show thatfor inflaton potentials $\propto |\phi|^{2n}$ near $|\phi|=0$ and flatterbeyond some $|\phi|=M$, the inflaton condensate oscillations can lead toself-resonance, followed by its complete fragmentation. We find that fornon-quadratic minima ($n>1$), shortly after backreaction, the equation of stateparameter, $w\rightarrow1/3$. If $M\ll m_{pl}$, radiation domination isestablished within less than an e-fold of expansion after the end of inflation.In this case self-resonance is efficient and the condensate fragments intotransient, localised spherical objects which are unstable and decay, leavingbehind them a virialized field with mean kinetic and gradient energies muchgreater than the potential energy. This end-state yields $w=1/3$. When $M\llm_{pl}$ we observe slow and steady, self-resonace that can last many {\ite}-folds before backreaction eventually shuts it off, followed by fragmentationand $w\rightarrow 1/3$. We provide analytical estimates for the duration to$w\rightarrow 1/3$ after inflation, which can be used as an upper bound (undercertain assumptions) on the duration of the transition between the inflationaryand the radiation dominated states of expansion. This upper bound can reduceuncertainties in CMB observables such as the spectral tilt $n_{\rm{s}}$, andthe tensor-to-scalar ratio $r$. For quadratic minima ($n=1$), $w\rightarrow0$regardless of the value of $M$.

Self-resonance after inflation: oscillons, transients and radiation domination