I think 10 hrs is more than enough to benefit them. I wont be sending lo for full 15 hrs when she is 3 if I can help it and will be fussy where she goes (only reason I will be sending her at all so she wont get a shock being away from me once she starts school, no other reason) I will be honest if they put school starting age to 7 I would be happy (and that is as a primary school teacher who when I worked was primarily teaching reception, year one and year 2) the children who I know who started at 7 did much better as they were ready. I know a lot of parents need it due to work so I feel for them as nursery is expensive, I am very lucky to be a sahm. I will be fussy where lo goes as I have worked with many in early years who just give sheet work when they should be learning through play.