Firstly, you'll most likely not be able to kill them by your own. So assuming you have 1 character to help you, the experience is doubled, but still only 5% of max until level 16.
http://classic.battle.net/diablo2exp/monsters/act1-cow.shtml
https://www.purediablo.com/diablo-2/diablo-2-experience/#Characters_Under_Level_25Roughly calculated
1,640,359 exp to level 25
806 exp each cow + 100% (1 extra character in area) = 1612 x 5% = 80,6 = 80 (afaik) each cow.
tel:1 640 359 / 801 640 359 / 80[/URL] = 20 504 cows. Minus about 20% since you get increasing bonus from level 16 = 16 403 cows.
According to this, there are about 350-500 cows in each run:
https://forums.d2jsp.org/topic.php?t=90178821&f=148&o=10tel:16 412 / 40016 412 / 400[/URL] (low avg) = 41 runs.
1 run takes about what .. 5 minutes on average if cleared with high level character? 41 x 5 = 205 minutes / 60 = 3.41 hours.
It doesn't seem so bad. My personal experience is that running from start and through all the acts is the fastest way to level up. Just kill monsters along the way. But that takes at least the same amount of time so, maybe it is easier your way.
But it will probably be more profitable to get level 20... 25 in hell, and then continue in a nightmare or normal.