I don't have a 1998 CR125, but my guess is either until it runs out the little screw you remove as a fill gauge, or until it shows in the center of the window on the gearbox... if not, I don't know
it should say on the casing somewhere round the top, mine is 750ml on a ktm 125 but there should be a bolt 2 check the level and it should just dribble out wen the level is right
I have a 92 cr125 and i fill the oil until it leaks from the screw on the side.The screw is the guide as too how much oil to fill. Id say around 600 ml's of oil.