They can climb down, jump the lava (they can do it with a couple of 5' jumps) and climb up the other side. They could also use ropes and grappling hooks, short distance fly abilities (for example from complete mage or the 4e sorceror abilities). So there should be a way they can get across.

As for time, about an hour (maybe a little more if you include the layout design and thinking time) to get to the point at the end of this tutorial I guess? The lava took a little longer. But if your players don't bite a plot hook involving a dragon's lair then you need to get yourself some braver players