I think children should start at the age of 3 in order to develop social skills and get used to being away from home, however at that age it should be something like 5-15 hours per week, so 5 would be my answer to your question (30+ hours per week).
However I think going to preschool at age 3 might be emotionally damaging for some children with a strong attachment to a parent, so it isn't for everyone, but I think it's a good rule of thumb for most.
What I don't think makes sense is going from 0 hours of school per week to suddenly 30 all at once, that is a heck of a scary transition.
So I think say in the year of being 3 years old you do something like:
First week: 1-2 hrs/ day, 3-5 hrs/ week.
First month: 2-3 hrs/ day, 5-10 hrs/week.
Rest of year: 3 hrs/day, 12-15 hrs/week.
And then build up from there to >30 hours by age 5 or 6.
There shouldn't be a sudden point when it goes from playing with toys in the sand to suddently rote learning maths and reading at a desk. The transition from play to learning ought to be a steady process. At the age of 3, it might be 90% play and social skills and 10% specific activities designed to build vocabulary, for instance. Children learn well through structured play and activities so there should not really be a clear dividing line between play and learning, either, I'd say.