Decent as in stainless or marine grade steel (or whatever material STW will argue is better) bearings with a quality sealing cartridge? I use Mobil XHP222 marine grease and very carefully lift the seals with a sharp, thin knife. I've repacked a headset about three times so far and still feels good.
I would say as many times as you want, provided you do it before they start feeling rough ie it needs to be a preventative measure, not reactive. Once they’re damaged no amount of repacking will help, although they might feel smoother short term. I guess no matter how careful you are there is the risk of the seal becoming ‘baggy’ eventually.
As timbog160 ^^^^
I've done my hope ceramic bearings loads of times. Been in constant use on different bikes since 2008. Still fine.
Plenty of times... 😉
