Tubeless road here - I'll assume MTB are the same. You should add sealant every couple months. You should also be able to run them at lower pressure than normal tubed tires. I run my road tires at 80-85psi and CX tires at 55psi.

If no one else has put sealant in them, do that. Otherwise ride and enjoy.

The sealant dries out so needs to be replaced from time to time (from every few months to yearly, depending on various factors). In case of puncture (which will hopefully be rarer) you just replace with a tube so flat kit is as normal. You can run your tyre pressures way lower giving heaps more grip. I usually go for around 23 psi

if there is a bunch of dried up sealant in the tire you might want to take that out and glug some fresh sealant in. Sometimes you need a compressor to reseat the tires after removal. Just carry a tube with you and if it flats remove the valve and install a tube like normal

Just be sure to spin the wheels once a week if you aren't riding it. I ride mine a lot in off-season but not so much in season and this seems to help the issue with drying.

Thanks so much everyone!

What size tube do I want in case of a flat?

Ask Slowman.

just a regular presta tube that would fit your tires is all you need. many people run tubeless tires and or wheels with tubes for a variety of reasons. For the MTB I really like tubeless a lot and always run some sealant even if its not needed just to help keep things sealed up and in case of a puncture is will often self seal.

I know some friends who carry a tire plug similar to one used on a car tire, I have no experience with these but they claim it's a good way to fix many flats without even removing the wheel from the bke

I run tubeless on both my MTB's - will never go back.

Having said that there are a few things you need to do... as mentioned, rotate the wheels if the bike is sitting, add new sealant every 6 months or so as it dries out.

Operationally - run less pressure in your tyres (tubes I ran 38, tubeless im 30-35 - 85kg), spares kit - take a tube and a CO2 canister or two... Whilst tubeless are less likely to get a flat, when you do... it'll be in such a way that you will have little/no hope of rectifying it on the trail. In this instance you will need to put a tube in it so you dont have to walk your bike home.

Sneaky trick - if you get a split in your side wall or something like that, stick a $5 note (Australian money is plastic.. not sure if it'll work with paper money) across the split and give it a few minutes for the sealant to react. You might be able to carry on as per normal.