Not sure, if you want a very specific answer from OP, but generally:

• it depends on the lentils you use (you want to use split lentils, but e.g. yellow lentils take a bit longer than red lentils)
• you practically cannot get it wrong. Just stir every so often to help them break down. I think, once they are broken down, the consistency doesn't change much anymore, even if you cook for longer.
• we're talking a range of 20-30 minutes, with somewhat chewy red lentil dal at 20min and creamy yellow lentil dal near 30min.
• As per usual, do taste it while cooking. You usually want a creamy consistency, but it's up to you.