How do you process leave in advance. employee taking leave for 6 week period?

Hi ckbookk, 

You certainly can process an advance leave in QuickBooks payroll. 

On the payrun that the leave will be taken, just go to Actions > Take Leave and enter the number of hours required.

Given the hours taken are more than the accrued, it will just reflect a negative on the leave balances and replenished as the employees accrues the hours on future payruns. 

I hope this helps. If you require further assistance or if you have a rather more specific case on this, please let us know so we can assist you.