The powder recycling practises for SLS is mainly based on the manufacturer's recommendation. The standard technique is to mix new powder with recycled in fixed ratios.
By measuring the MFR (Melt Flow Rate) of PA12 powder you can evaluate the recycled powder deterioration and control its quality. MFR for fresh EOS PA2200 - PA12 MFR is in a range 50 -60 g/10 min. It is considered that powder with MFR below 20 g/10 min will not give satisfactory surface quality. Time and temperature have been proven to be the most influential parameters, not the number of powder recycling.