Clean, 255 bytes

import StdEnv,StdLib
a=[-2..2];b=diag3 a a a
?m=iter(size m*2-1)\p=:(x,y,z)=case m.[0]of'F'|z>0=(y,~x,z);'U'|y>0=(~z,y,x);'R'|x>0=(x,z,~y);'B'|z<0=(~y,x,z);'D'|y<0=(z,y,~x);'L'|x<0=(x,~z,y);_=p
$l=length(takeWhile((<>)b)(tl(iterate(map(sseq(map?l)))b)))+1

Try it online!

Clean, 255 bytes

Derived separately from the almost-identical Haskell answer as an answer to this question which was closed as a duplicate when it was almost finished, so I posted the answer here.

import StdEnv,StdLib
a=[-2..2];b=diag3 a a a
?m=iter(size m*2-1)\p=:(x,y,z)=case m.[0]of'F'|z>0=(y,~x,z);'U'|y>0=(~z,y,x);'R'|x>0=(x,z,~y);'B'|z<0=(~y,x,z);'D'|y<0=(z,y,~x);'L'|x<0=(x,~z,y);_=p
$l=length(takeWhile((<>)b)(tl(iterate(map(sseq(map?l)))b)))+1

Try it online!