It would take approximately 400,000 hours or 45.71 years to cremate 6,000,000 bodies with 15 crematoriums.

Calculation:

Number of hours per cremation = 1 hour

Number of crematoriums = 15

Number of bodies to be cremated = 6,000,000

Total hours required = Number of bodies * Hours per cremation / Number of crematoriums

Total hours required = 6,000,000 * 1 hour / 15 crematoriums

Total hours required = 400,000 hours

Converting to years:

Number of years = Number of hours / Hours per year

Number of years = 400,000 hours / 8,760 hours per year (assuming 24 hours per day, 365 days per year)

Number of years = 45.71 years (approximately)

The Holocaust was a hoax