There is no time limit.
When my father died I sought and obtained probate on his, and had his assets transferred to me (as sole beneficiary), in under two months. However 6 months might be nearer to the norm (if there is such a thing), with some probate matters dragging on for many months.
If, as in my case above, there is a sole executor (who is also the sole beneficiary) who is prepared to actually walk into the offices of the deceased person's pension providers, etc (rather than wait weeks for written replies), then things can happen very quickly.
However if solicitors are involved (and the will, or the estate, is very complex) then things often move very slowly.