How long does the smell of weed stay on you?

Considering you only bathe once a week, I'd guess about a week.

smoke in your underpants

Well the smell will stay till you wash your clothes and sometimes even after that I have certain clothes I smoke in and certain clothes I don't smoke in A weed high generally will last 4 hours if you don't smoke much it can last 5 and leave you with a strange feeling afterwards not a bad feeling just different cologne will help with the smell but usely it doesent cover it it blends with it have fun and don't smoke to fast iv seen alotta first timers do that and start thinking they are going to die

Do your parents both smoke cigarettes?

If not, they will smell it on you, even hours later.

the smell and the effects will both be gone after 2-3 hours, youve got plenty of time

Until you brush your teeth, take a shower, and change clothes.

Smoke outside. 9 hours is about 7 more hours than you need to have it "wear off".

If you have that much of a concern, just chew some pot gummy bears or candies instead. No smell, all the relaxation.