Starting with the 15th day staying in the same location, the daily allowance is being reduced by 50% for trips within Germany, and 10% for trips abroad. Travel days (there and back) don’t count towards the 14 days. Hotel costs will be reimbursed even after 14 days, but you have to show proper receipts. Attention: regulations may change depending on the financing of your travel!
There are special regulations regarding trips longer than 4 weeks.