Valium is a long acting Benzo and it will stay in your body for a couple days, but stop working after about 10 hours (it just depends on the person).
If dosed appropriately for size and weight, the effects can last for 8 to 12 hours.
In the short term, the physical symptoms will last no more than half an hour, as your body produces a hormone which blocks the adrenaline. Long term, you can suffer with anxiety for years (unfortunately!!) Seriously, don’t be afraid to seek professional help for your anxiety – It’s so so common.