ANSWER:

Once the flow of the blood stops, you must not perform ghusul until you see the white thread like discharge or see complete dryness. 

This dryness is established by inserting a tampon and if it comes out clear, this is a sign of your purity.

QUESTION: In 6th/7th day of period, after being clean for more than 12 hours (which is one of the sign of purity) I usually have a very mild flow for 30-40 minutes.The flow is too mild to leave any stain on my cloths, yet during this time if I check with a tissue, it won’t come clean.Should I treat this flow as spotting?