As far as I know a rider can sign a temporary contract for 28 days so that the rider he is replacing can come back in to the team on his previous average, I don't think it is a minimum, it is just done to protect the average of the unavailable rider but I think you can sign anyone for as long as you like but if he is not needed then you will have to rebuild the team to the team limit or replace with a rider on a lower average.