the answer to that really depends on where you are on the planet, and what your growing. for example, full sun plants like peppers and tomatoes can typically only handle about 6-8 hours of sun at my house.. but up north where you are i bet 10 would do much better. especially with the cooler nights.
being where you are i bet moving them outside would be just fine though. have the plants been "hardened off"? you need to do that first if you havent already. other wise the sun will burn em.