When I recently bought my new MTB: CUBE ACID 29" 2017, the shop put sealant liquid in the inner tubes. So far so good, great, no punctures, no loss of air, really happy. My questions: How often (or mileage) do I have to add liquid? Do I have to replace the inner tube? Any other recommendations about that? Thank you all!
It depends. If you have regular tube tires and the sealant was something like Slime, then in theory you don't have to put more in unless you have a flat. In reality, Slime thickens over time. You can't really add more, so you might expect to change tubes every couple years. Riding your bike regularly will help prevent the Slime from coagulating in one spot and causing an unbalanced tire.
If you have tubeless mtb tires, and used something like Stan's then you should reseal every 2-6 months according to the manufacturer, more often if you have punctures or loss of air.
In either case, you want to keep the total volume of sealant the same over time. Which means you do not want to add more unless some has leaked out or you're able to remove it through other means (this is easy with tubeless tires; it's near impossible with tubes).