17:57 May 16, 2011 

I'm not so sure that loyalty is the right term here - loyalty implies some sort of choice by the employee or, as you say, working longer for a particular customer than is normal. Whereas I think the agencies have to pay these Zuschläge whether they want to or not (as part of the Tariff) - this is one way of narrowing the gap between cheap temps and costly long-term employees (from the unions' point of view); or ensuring the individual's pay gets closer to those lucky enough to be in full-time employment (from the temps' point of view).

17:27 May 16, 2011 

bonus obviously depends on working longer a particular customer than is normal. Sort of a customer loyalty bonus as viewed by the temp agency.

06:56 May 5, 2011 

I'm thinking along the lines of:

If Mr/Mrs A. N. Other works for the same customer for a period of X calendar months without interruption, a einsatzbezogener supplement/bonus of X.X% shall be paid.