Questions:

(a)
How much has CO2 concentration in the atmosphere increased since beginning of industrialization age, let's say 1800

(b)
Take the amount of CO2 produced during one year by non-natural processes (=human activity).

Maybe difficult to say what that is exactly is, humanity also burned natural materials for "primitive" heating and nightly lighting since the stone-age. So count any "high level activity" beyond primitive use of fire.

How many years do all natural reduction processes together (algues in the ocean + trees and plants on land) take to decompose this supplemental CO2?

Otherwise formulated:
if humanity could stop NOW any combustion of fossile energy sources, how many years would the CO2 concentration reversal to a year 1800 level take?

Would like to see scientific estimations about that... or a link to scientific pages about that (?)

Roughly 190 Giga tonnes of CO2 is emitted by natural processes annually and only about 8 Giga tonnes is emitted by human activities. The atmosphere contains approximately 750 Giga tonnes of CO2, roughly a quarter of which is reconstituted. This amount is increasing at a rate of 3.5 Giga tonnes annually.