Topping up should be done, at least, according to the manufacturer’s recommendations. Every 100 days for my set up. If you are at 120 days it is time to top up.
I maintain a number of bikes [my own and my group] and have some experience on topping up sealant.
Having used other sealants I have found Effetto Mariposa the best sealant. If you want to double up on sealing power just add a little fine glitter. That will bung up any larger nicks or gouges.
If you have a good rim [which you don’t specify] topping up should be as easy as deflating the tyre, removing the valve core, and injecting the sealant using one of the syringes on the market. Road bikes usually need 30-40 ml and MTB around 60 ml.
WARNING: If you are running one of the modern *narrow-shoulder tubeless-ready rims you may find the tyre wants to slip off the bead into the channel of the wheel, when you deflate to top up. It will then be infuriatingly difficult to refit the tyre with fresh sealant as the wet inner rim simply can’t hold the tyre in place to inflate. Even with a full-flow compressor it can be impossible to get the tyre back on to the bead to seal. One of my mates took his to his *professional bike shop/dealer to top up but they had to install new tyres on his Bontrager road wheels. Another running 2019 GIANT carbon rims found the dealer had to install tubes because the tyre would not stay on to seal on top-up.
Fitting a FRESH new tyre on a CLEAN dry rim is usually fine. A moist rim lets the tyre slip off the bead into the channel if you are running one of the narrow shoulder profiles. Giant, Bontrager and other modern road rims I have found very problematical on sealant top-ups. So be careful and prepared for this issue.
On the other hand, ENVE’s rim profiles are superb. After years of topping up with sealant I have never had them slip off. They are expensive but they are superb on this score.
I have never had a flat running tubeless though I have had loads running clinchers.