This is a question for which there really is no definite answer. There are just too many variables involved. In order to answer this question, we would need to know what is the composition of the lava, how hot is it when it erupts, how big is the lava flow, and what is the shape of the flow. The first two criteria will determine how viscous the lava is. (The higher the silica content of the lava, the more viscous it will be. The hotter the lava, the less viscous it will be.) The last two criteria will determine how quickly the lava will cool. (A large flow will cool more slowly than a small flow will and a long, thin stream will cool faster than a massive pile.) Because of these variables, it is impossible to really give an exact answer to this question. Some lavas emerge from the vents viscous enough that you could walk on it. Others can flow for tens of miles before they cool enough to walk on. Also, you should not confuse being able to walk on a lava flow with the lava having solidified. As a graduate student I took a field trip to Hawaii. On that trip my classmates and I had the opportunity to actually examine an active lava flow. A group of us hiked out to that flow at night. It was a relatively small flow and had cooled enough that it had crusted over. So, we could only see it glow in place where the crust was broken. Therefore we had to really pay attention to where we were walking lest we step out onto the flow. One of my classmates wasn't paying attention to were he was walking at one point and did actually walk out onto the flow. He didn't realize that he was standing on molten rock until we told him to get off of the flow. He was a very obese man, yet he did not sink into the flow. He did manage to ruin his hiking boots, but was otherwise unharmed by his excursion.

That depends on how thick the lava flow is and how cool you want it to be. A thin pahoehoe flow is cool enough to support a person's weight in only 10 minutes or so, but it is still hot (and still molten inside). For it to completely cool, a flow that is about a meter thick will probably require more than 2 days. This is because lava is such a good insulator. An extreme example is a lava flow that was erupted in 1959 and flowed into aa pit crater. Because this lava flowed into a depression it got very deep. In fact, it was about 85 meters thick. It was drilled in 1988, and there is still some mushy, not-quite-solid stuff down near the bottom. Scott Rowland, University of Hawaii.

Lava cools slowly because lava is a poor conductor of heat. Lava flows slow down and thicken as they harden.

I'd say about 4 or 5 hours.

I think that it takes a matter of minutes I just watched a video and it showed the lava moving and the lava went solid as soon as it got slower and then after it cools enough then you would be able to walk on it.

Pretty much instant. Have you ever watched footage of lava pouring through water? Insane.

Here is a similar question (http://www.answerbag.com/q_view/21448).