We explore the fate of the Universe given the possibility that the density associated with `dark energy' may decay slowly with time. Decaying dark energy is modelled by a homogeneous scalar field which couples minimally to gravity and whose potential has at least one local quadratic maximum. Dark energy decays as the scalar field rolls down its potential, consequently the current acceleration epoch is a transient. We examine two models of decaying dark energy. In the first, the dark energy potential is modelled by an analytical form which is generic close to the potential maximum. The second potential is the cosine, which can become negative as the field evolves, ensuring that a spatially flat Universe collapses in the future. We examine the feasibility of both models using observations of high redshift type Ia supernovae. A maximum likelihood analysis is used to find allowed regions in the {m, ₀} plane (m is the tachyon mass modulus and ₀ the initial scalar field value; m ~ H₀ and ₀ ~ M_P by order of magnitude). For the first model, the time for the potential to drop to half its maximum value is larger than ~8 Gyr. In the case of the cosine potential, the time left until the Universe collapses is always greater than ~18 Gyr (both estimates are presented for Ω_0m  =  0.3, m/H₀ ~ 1, H₀  70 km s⁻¹ Mpc⁻¹, and at the 95.4% confidence level).