My wife and I are trying to negotiate a separation and it is not going well. I could see her packing a bag and leaving the marital home for several days at a time, if not longer. At what point does this constitute abandonment? How long does she need to be gone & without contact before I can legally change the locks on the house? I don’t want to be accused of maliciously turning her out of doors.

Abandonment only matters with regard to alimony. If she leaves for a significant period of time, or pursuant to an agreement about separating, you can get the locks changed. If she returns, you can enforce the domestic criminal trespass statute.