First let's get this term 'top-up' out of the way. Although this expression is often used, I try to avoid using it as in common with any self respecting aircon technician, a top-up is never just a top-up.
To operate efficiently an AC system needs a specific quantity of refrigerant. For example a 1990 BMW 525 needs 1,925 grams of R12 whereas a 1999 Rover 200 uses only 560 grams of R134a plus or minus 25 grams. In other words for optimum operation this Rover needs somewhere between 535 grams and 595 grams, no more and no less.
This is perfectly typical of modern cars - if the system had only 510 grams or 620 grams it would work slightly less effectively and if the disparity were greater the efficiency would fall even more, indeed if it had too much refrigerant it would quickly get to the point of damage to the compressor.
Now to get back to the 'top-up', if the quantity of refrigerant is so crucial, how do I know how much refrigerant to add to the system to bring it to the correct charge? The short answer is I don't. Except in only exceptional cases for short term expedience would I consider just adding refrigerant to an existing system - the correct procedure which is followed in 99.9% of cases is to recover all residual refrigerant, evacuate the system to deep vacuum and recharge with the correct weight of refrigerant.
The evacuation to deep vacuum plays an important part in the recharge; it is also the lengthiest part of the recharge procedure. The object of this is to achieve such low pressures within the system that any moisture that has become adsorbed by the hygroscopic oil will boil away at ambient temperature and be drawn away by the vacuum pump (remember that water boils at lower temperatures as the pressure is reduced - water will boil at only 10ºC at a vacuum of 29.66 inches of mercury and increase that vacuum to 29.76 and water will actually boil away at the same temperature as it normally turns to ice).
What happens in systems where there is no published recommended charge weight? For example where a system has been modified for any reason or a bespoke system made up. In these circumstances the AC technician must use whatever visual and tactile signs he can detect to get the system operating correctly. With conventional compressors this is not too difficult but now that Variable Displacement Compressors are so common in European cars it would be easy to get it wrong.
absence of any other indications (ask yourself the questions on the page
above) a reasonable rule of thumb would be for a first recharge when the
car is four years old and thereafter every three years. It is certainly
cheaper (and more comfortable) to keep a good charge in the system rather
than to have to repair a system that has been allowed to deteriorate with
a depleted system.