How long till iron tablets should effect stool (i have been off them for two days)?

Depends...: How fast your stools normalize off iron depends on how fast your gut moves through the iron pills you've taken. Most people have a GI transit time that permits a stool frequency in a normal range of 3 movements/day to one movement every 3-4 days. Therefore, if you're still seeing dark stools after four days of use, or symptoms warrant, then further investigation of those dark stools is in order.