Shampoo

Whenever your scalp is oily or itchy. Daily is fine if your scalp is oily; every three or four days suits a dry scalp. Deliberately "training" your scalp to produce less oil by not washing doesn't work.

Better for dry scalps, coloured hair and daily washers. Otherwise it's just gentler cleansing, which isn't automatically an advantage.

Shampoo for the oily scalp, conditioner and a weekly mask for the dry ends. That combination is extremely common and needs two different products, not one compromise.

Usually you use it until it clears, then a couple of times a week to keep it away. Dandruff is managed rather than cured, and it tends to return if you stop entirely.

Give it three or four washes. Switching from a silicone-heavy formula in particular can feel worse before it feels better, because you're seeing your actual hair rather than a coating.