How long does it take before i know if i have bed bugs?

About a week: About one week after sleeping in the suspected spot....

Bed bug bites may: Cause no sx's/signs in some except for 2 small points where skin was punctured. When there's a reaction, it may show as fast as an hour or may not show for several days after being bitten. It can cause itchy, pink, swollen, lesions with a clear center. In cases of hypersensitivity blisters may develop. A telltale sign is linear track from sequential feedings – but doesn't always happen.