At the current rate, Frieren will probably end some time in the mid 2030's.

The Manga has been around for 5 years this April, and has produced 140 chapters. Given that the anime seems to desire 60 chapters per season, it makes sense that the author is aiming for a full series of either 240 or 300 chapters.

Chapter production has slowed considerably since the Anime was announced, as production of a show takes away time that could be spend on producing new chapters. The chapters themselves also take much longer to make as a combination of more attention being given and a general slowdown of what was initially a rapid fire production pace.

Given the above, I estimate that the manga will not reach a conclusion until at least 2032, if not later.

At the very least, once Season 2 of the anime is complete they will need to release at least 40 more chapters before they can start on the next season. At current rate that will take 18 months, starting around next September when the second season will likely be released.

If I had to guess, I would estimate the show will have either 4 seasons and a movie or 5 seasons, and the final episode will come out some time in 2034/2035.