So, I've setup my first tubeless ever 3 months ago, using Orange Seal Endurance.

According to Orange Seal, I'm approaching end of the sealant lifespan (120 days).

What shoud I do now?

a) just remove valve cores and add some more sealant? How much?

OR

b) take the tires off, clean tires and rims from sealant remains and set it up again from scratch?

So far, usage experience was excellent. No punctures whatsoever, and setup on Mavic UST wheels was a breeze.

Please note, as this was my first setup ever, I've used quantities from fhe upper reccomended range (approx. 60 ml per wheel).

Wow, that's a long time - I've learned to check my sealant every 6 weeks or so, it can be problematic otherwise.

Just check the level - put the bike on a stand, deflate the tires, rotate the valves to the 6 o'clock position, remove the core and use the dipstick that comes with the 8oz bottle. If it's low, top off.

I have been using orange seal for two years. I periodically top off as WhyFi recommends, although not anywhere near as frequently — every 3or 4 months. Honestly, it only ever occurs to me to check when I notice the tire seems unusually low after a few day since last pump up and that doesn't happen often.This thread reminds me that I am probably due for a check.

I used to check at much longer intervals, but I've learned that my flat frequency can exhaust the sealant in as little as 8 weeks or so, particularly on the rear tire, at the height of my flat season. It sucks to find out that your tire is dry mid-ride. Since my bike is stored on a wall-mounted stand, it's a matter of a few minutes to deflate, remove the cores and check the sealant levels - a minor inconvenience compared to breaking the bead, inserting a tube and using a mini-pump on the side of the road.

YMMV, obv - if my experience told me that I could get away with 10 or 12 weeks, I totally would.

No idea what size the tires are, so no idea how much sealant you should put in. I start (initial fill) with 45mL in 700x25, and 60mL in 700x35. Refresh is 30mL and 45mL respectively.

Each of my bikes puts on 500-600 miles a month, so sealant is usually* getting topped off around the 1,500 mile mark, which happens to usually correspond with ~90 days. I have a chalkboard near the bike stand to remind me.

Don't mess with taking the tires off and cleaning them out, unless you have had many, many punctures and suspect some coral has formed. I ran a tire for 13 months (5,900 miles,) and did nothing but periodically refresh sealant-- it probably had 400mL of OS through it. When it came off the bike, it was 45g heavier than a new one.

*Climate is an important factor in sealant life. I go through the fall and winter on one shot of sealant, but in the summer, when the pavement can hit 180º, sealant is sometimes depleted in as little as 3 weeks.

On some deep Mavic wheels (Comete UST) the valve is actually a valve and an extender, in which case the dipstick method won’t work as the dipstick won’t fit through the tiny hole. Gotta pop the bead then to check (although not exactly a big deal).

Orange seal used to make a dip stick that they included or still include with the small bottles. Remove the core and insert the dip stick and check the level.

In our area you should refill every 3 months or so (straight from the mouth of Stans) while other areas can usually get 4 months. He said because of our cold. Meh. Just add. At some point when you have nothing better to do or keep having valve fouling problems then yank the tire and pull the "stanimals" out.

There is no reason to pull all the skin off from the inside of the tire. It won't hurt anything if you do but the new sealant you just put in will end up having to coat the tire again anyway.