In the case of a long-term horizontal load you would use a drained coefficient for the interface for which tan delta could be 0.3-0.35.
For short-term loads, undrained strength and adhesion...I have seen this taken as 1 (i.e. adhesion = Su) or as 0.5-0.8 as HTURKAK suggested for a sliding check on shallow foundations, although I personally don't see why it should really be any different from using an adhesion factor for an undrained pile analysis (where you have 1-1.1 for a low Su of 20-25kpa which scales down following a linear or curved trend to say 0.2-0.3 for Su=200 kPa, the problem is just rotated in a different direction.
