LengthWhile

LengthWhile[list,crit] gives the number of contiguous elements ei starting at the beginning of list for which crit[ei] is True.

Examples

Count elements less than 4:

LengthWhile[{1, 2, 3, 5, 2, 1}, # < 4 &]
(* 3 *)

Count positive elements from the beginning:

LengthWhile[{1, 2, -3, 4}, Positive]
(* 2 *)

Please visit the official Wolfram Language Reference for more details.