It will stay suspended as long as it's on the plate, but you will see CO2 bubbles rising as an indication of fermentation with white bubbles collecting on the surface and in the vortex made by the stir bar spinning or sometimes you will see a krausen on the surface, but krausen does not always happen with starters. Regardless, the presence of rising CO2 and the formation of foam shows the existence of fermentation which gives you viability.
I usually spin it for 48 hours, then refrigerate overnight. This is where you see the new white cells that reproduced. Then decant at brew time and allow the slurry to warm.