What would the precursors to a repeated Lake Toba super eruption?
Massive, caldera-forming eruptions like the one at Toba usually happen at large silicic systems. At such systems, we expect a surface uplift as the pressure builds in the magma reservoir. This can be measured by radar interferometry (InSAR). It is the case at Laguna del Maule volcano (Chile), were uplit up to 28 cm per year has been measured by InSAR (Feigl et al. 2014). A similar pattern would be observed at Toba if a magma reservoir was being replenish. However:


We don't know the timescale needed for overpressure, thus eruption, of the reservoir. It depends on many parameters, such as reservoir size and depth, strength of host rock, rate of fresh magma supply...
We don't even know if such uplift will lead to an eruption. The magma could very well be intruded into the crust (in the form of dikes and sills), releasing the pressure in the reservoir. The uplift would then cease without an eruptive crisis.