How long does it take for trees to leaf out in the spring?

Individual trees can leaf out very quickly. You can hear it happening on a quiet night. But different varieties leaf out at different times. Norway maples are very early and you might think a sycamore is dead because it leafs out so late.

Depends on the tree, the vegetation or whatever. They do it at different rates and time. Takes about a month for my maples to first start showing signs of their blossoms until they are totally leafed out. My grape vines on the other hand, take things much slower.

I'm certainly no botanist but from my experience once the trees start to bud it happens fairly quickly especially when its sunny and warm. Cool and rainy days will delay them a little bit but by the end of May most are in their summer form, at least in southern lower Michigan. 

about a week or so

It depends on the soil temperature. Once it warms up enough, they start to leaf out.