Hi Kadin,
the duration depends on the temperature you start out with, and the amount you put in. The more material you have, the longer it takes for the clay to be warmed evenly throughout.
Generally, go with the lowest setting on your oven and check back every 10 to 15 minutes. The clay should become malleable but not melt. And be careful, don't poke your finger in. In case the core should have melted you could seriously burn yourself!
Happy sculpting :)