Jamieson, Fausset and Brown on 1 Samuel 29:3

3. these days, or these years--He had now been with the Philistines a full year and four months (1Sa 27:7), and also some years before. It has been thought that David kept up a private correspondence with this Philistine prince, either on account of his native generosity, or in the anticipation that an asylum in his territories would sooner or later be needed.