Hi gb

Above a certain frequency water is dissociated quite efficiently by UV light. UV light is about 6% of the solar spectrum. If 1% of the total flux is high enough, then some ~25 W per square metre of stratosphere is going into dissociation. It takes roughly 460 kJ/mol of water to bust it up - about 25 MJ/kg. Thus about 1/1,000,000th of a kg is dissociated per second per square metre. Venus's sunwards face has an area of 1.2E+14 square metres, thus some 120,000 tons of H2O can be busted up per second. That's 3.8 trillion tons per year - if the stratosphere is wet enough. Assuming that then it'd all be gone in less than a million years if there was enough of an outward flow of hydrogen to stop it from recombining.

The limiting factor is the hydrogen loss rate and the mixing fraction of the water in the stratosphere not powering photolysis itself.