I think it's a matter for individual parents to decide, rather than there being an optimum, "recommended" time. What you've described sounds pretty reasonable, especially if it's balanced by plenty of "real world" interaction, whether in sporting or other activities.
Your kids will of course insist that Johnnie or Sarah's parents (or "everyone in school"'s parents) let them have much, much more. As with other such pressures, stand firm and explain to them that you're not Johnnie or Sarah's parents and that you're sure they wouldn't expect to follow your house rules, any more than you intend to adopt theirs. I'm not one for Luddite, "the-internet-is-the-devil" stances, but I do think that reasonable curbs are necessary given the extremely addictive nature of the beast (and internet addiction is very far from being restricted to kids, btw!)
The effects of excessive online activity on young kids' mental and social development are only beginning to be reliably documented, IMHO. But ask any teacher what they think.