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This question cannot be answered with certainty.  You should appreciate that sodium hypochlorite solutions are only stable at around 12-13% in water with sodium hydroxide.  Try to concentrate it more than that (say, by evaporation on the roof) and it will decompose rapidly to that concentration.  The rate at which the active chlorine is decomposed or used is also affected by sunlight, temperature, oxidizable soils and organic compounds, transition metal compounds and pH.  So the rate at which the active chlorine (or hypochlorite) is diminished with time depends on a lot of factors.

Sodium hypochlorite is manufactured by bubbling chlorine gas through a solution of sodium hydroxide solution and much of the sodium hydroxide remains behind in the concentrate you purchase from your wholesaler.  I am not a trained roof cleaning contractor and I haven't yet treated even a single roof, but I would think you should rinse all of the cleaning solution off of the roof before you leave.  The residual caustic can cause severe damage to the shingles as well as any plants under the conditions you describe.