Right, I've been running my Schwalbe One tubeless for around 6 months now. Is it time to refresh the sealant? And does one just add more through the valve, or should one dismount the tyre and remove the (residue of) the old?
The sealant stays liquid, and when you get a puncture it is expelled through the hole, turning solid as its temperature drops. A while ago in my early days of tubeless, my LBS changed a worn out tyre, and as they were also in the learning stage, they cut off a quarter of the old tyre and counted the tiny 'plugs' where the tyre had been penetrated and the liquid had sealed the hole. There were about SEVEN plugs, amazingly, in a quarter of a tyre. I imagine they chose the section of tyre that looked to have the most, but even so that's clearly a lot. And even if the other sections of the tyre had less, it's clear that LOTS of penetrations had self sealed, unnoticed by me. Each time this happens you ride on, but you lose some air and some sealant....
So if you have had a few 'unnoticed', you will also have lost liquid.
Also every time you pump more air into the tyre there is likely to be some sealant around the valve. When the valve opens as the pump pressure exceeds the tyre pressure you obviously get a flow of air into the tyre, but that agitates the liquid and some becomes gaseous, and then when your pump stroke finishes some of that mixture leaks back through the open valve into the pump. When you eventually disconnect the pump you get a tiny hiss as the higher pressure within the pump tube escapes, and in that 'hiss' there is usually a tiny amount of vaporised sealant. So you lose tiny amounts of sealant just about every time you pump up the tyre. (So it's good practice to set the valves on both wheels to the top when you 'park' the bike, to allow the fluid to drain away to the bottom of the tyre....)
Definitely time for more sealant after six months, I should say. Tyre might well be dry now....
I have a syringe bought from Stans No Tubes that allows you to put in more sealant provided you first remove the valve core, but you lose all the air, and you then have to reinflate the tyre, which is easy if it's sealed itself to the rim, but can be an issue if it pops off the rim and you have no compressor...
If it's sealed itself to the rim then no need to dismount. If the tyre pops off the rim, then remove it and scrape out the old solid stringy bits that had formed the seal. These stringy bits are no use once you break the seal.... After you put in new fluid and reinflate quite a bit of the fluid escapes around the tyre/rim, hardening and sealing as it goes, and that's the basis of the stringy stuff you find the NEXT time you take off the tyre....