Aircraft hydraulic is a very complicated system with a lengthy plumbings, joints, connections, valves, pumps and so on. As the aircraft aging, especially on component where the seals start to deteriorate, leaks can occur. This is most common findings during routine inspection on aircraft especially in the wheel well area.
In the maintenance manual, the allowable limit basically is how many drop per minute. It further will be specified static or dynamic which is with component is off condition or operational. Aircraft engineers will switched on the relevant aircraft hydraulic system and observed the leaks to find whether it is within limit or not. If out of limit then the component must be changed before next flight.
Not all leakage is allowable, on plumbing connection for example leakage is not allowed at all. With 3000 psi pressure (that is normal operating pressure for most aircraft) any leakage can drain whole system fluid, the good news is that normally there is 2 or 3 independant hydraulic system available on an aircraft so if one system is down, there is still a backup.