If I take 5mg of diazepam at 8am can I drink by 6pm?

Generally speaking the answer is no! However, the best person to ask this question to is your doctor who prescribed the diazepam to you. If you read the pamphlet that comes with the diazepam from your pharmacy it will say "do not drink alcohol while taking this medication. "
I strongly recommend you talk with your doctor before you drink any alcoholic beverage while taking diazepam. You can also ask your pharmacist where you get your diazepam. The combination of alcohol and diazepam can be lethal.