Depends on a lot of factors, bud mass, climate, airflow... I've had to dry for as long as 2 weeks. I go by how springy the buds are when squeezed, how dry the stems are... When bud is in jars, you don't want humidity any higher than 60%. It's easy to tell, if when you pop the jar the air inside is warm and moist, take the buds out for an hour or two, then close back up. Never had mold issues.
The hygrometer is really useful, once the humidity in the jars equals your ambient humidity, drying stops so you have to be careful to open the jars maybe every couple of days once they get between 60% and your ambient humidity. At 63-65%, I'll just leave the bud in jars with the lids off till they hit 60%, but that's just me. Hygrometer/thermometer can be found at walmart for $6.