It depends on how close you are, what kind of lava flow it is, and whether you are upwind or downwind. For example, the most approachable lava is pahoehoe. This is because each toe forms an insulating skin seconds after emerging on the surface. This skin is at first flexible and then hardens, but even when flexible it is a good insulator. This serves to keep the interior of an active pahoehoe toe hot and fluid but also prevents you from getting burned by the radiant heat. If the wind is at your back, you can easily approach long enough and close enough to get a sample with a hammer. It is still hot, and unless you are well-protected you can only be that close for a minute or so. You also notice that as soon as you peel the skin off to get at the molten interior, the heat goes way up. This is heat that you can’t stand, you have to get back otherwise blisters start to form. It is hot enough that you can’t accidentally step on active lava.
Skylights into lava tubes on pahoehoe flows are quite hot, and have to be approached from upwind. They are so hot that the air shimmers over them so they too are hard to miss. They are dangerous not as much because of the radiant heat from the lava inside but because of the super-heated air coming out. You have to be really careful that the wind doesn’t shift, and many a volcanologist (at least this one, anyway) has gotten singed hair, eyebrows, etc. when the wind changed.
An ‘a’a flow, on the other hand is terrible to work near. Instead of a relativel continuous skin, ‘a’a flows have discontinuous layers of clinker, and a huge amount of radiant heat escapes from between the clinker. ‘A’a flows also move faster so you really have to be quick on your feet if you want a sample. Additionally, ‘a’a flows tend to form open channels rather than lava tubes. The channels can sometimes have completely incandescent surfaces because they are flowing so fast that any skin that forms is immediately torn or sunk. Even flying over a large channel in a helicopter 200-400 meters above the flow, you can feel the radiant heat from the flow through the windows!
When Hawaiin lavas erupt, they are around 1170 to 1175 degrees centigrade. (about 2140 degrees farenheit) Other volcanoes produce cooler more viscous lavas in the 700C (1292F) range. (Still much hotter than in your oven when a turkey is baking!)