Recharging or renewing?
A sphere is only as good as the neoprene membrane that contains the pressurized gas. Occasionally, the interface between the membrane and the retaining plate inside the sphere is to blame for leakages, but usually any problem is associated with the membrane itself. Like anything that's under constant flexing, it'll break down over time.
When the internal pressure drops below about half the original, the reduced gas pressure could allow the membrane to start smashing against the inside surface of the sphere, adding to the already high stresses. If there are any imperfections on the inside of the metal, that's a further detriment to the membrane's life expectancy. Several people who recharge spheres won't recharge them once they've dropped below about half pressure because then the incidence of failure is too high—leading to wasted time and gas, as well as irate customers.
However, many spheres seem to go on indefinitely—they lose very little pressure over many years' use and then remain in good working order for many more years after being recharged. On the other hand, some spheres don't last any time at all (I had two new rear ones from a factory-installed bad batch of neoprene that lasted one or two days before losing virtually all their pressure, leading me on a wild goose chase suspecting everything else in my car other than the new spheres).
On balance, I say you are far better off recharging for as long as the spheres retain their pressures reasonably well. Once they start dropping pressure too quickly, chuck them and buy new. What helps greatly in all this is access to a pressure tester—a simple device using an old hydraulic jack, a pressure gauge and an old Citroën cylinder into which to screw the sphere to be tested. Then it's an easy job to test your pressures on a fairly regular basis and thus keep tabs on what's happening. You can also tell by the ride quality of the car and how soft it feels when you push down on the bumpers, but the testing rig gives you definitive numbers.
Theoretically, the best thing to do would be to keep the spheres always at the original pressure. This is of course not feasible but if you, say, refill your spheres as soon as they lose 10-15 bars of their pressure, you'll be close. The point is to refill them early on, long before they even start to go flat. First, when a sphere goes real flat, your chances of successful refilling diminish (you will usually be able to refill them but they won't hold the pressure long). Second, if you allow them to drop their pressure, this means that you have used your car for quite some time with reduced pressure—bad for comfort and other parts in the suspension. So, from a technical point of view, frequent (every 12 months or less) refilling is the best solution, always keeping the pressure in the best and most comfortable operating range and ensuring a long life for the spheres.
However, finances often say different. If you don't have easy access to pressure checking and refilling equipment and garages doing it charge you more or less the same for refilling as you would pay for new spheres, you probably want to go for new spheres, anyway. But you certainly will wait longer before replacing the spheres, you'll let them go flat before you do (I don't think you would want to replace a sphere when it is only 10-15 bars below original).
To summarize, to refill the spheres successfully, you should do it much more often than you would consider changing the spheres acceptable. Failing that, there is not too much point in refilling.