When $\mbox{RU}$ coming back to me?

Now, let's spice things up a bit. In the last problem , we did $\mbox{R}_2$ and $\mbox{U}_2$ moves to transform the cube. This time, let's do $\mbox{R}$ and $\mbox{U}$ moves (only one clockwise quarter turn each time). If we perform $\mbox{R}$ followed by $\mbox{U}$ , we call that the $\mbox{RU}$ permutation.

How many $\mbox{RU}$ permutations must we perform before the $2\times 2\times 2$ cube is back to its original state?