I Like This Series, How Long Can It Continue?

In history, we are taught she remarried for quite some time and remained in Scotland. While it's quite evident the people running the show are not following all the facts. Reign is more loosely based on History.
Personally, I believe they have many more Seasons if they so choose. I am a fan of this show. I am very hopeful they give a bit more respect to Reign.
It feels as though they don't care if it succeeds like they use to. We shall see.

Historically she never did return to France.

In terms of writing for TV, seeing the craziness they made out of her life in France, there's still plenty to work with. Things were very tumultuous when Mary went bank to Scotland. At this point, it's late 1561, since she returned in the summer of that year, presumably to determine if she was carrying Francis' child (he died in December 1560). Mary remarried in 1565. Her son was born in 1566. She miscarried her second husband's twins while imprisoned in 1567, just over five months after he was murdered and two months after marrying the man believed to have killed him. She fled from Scotland in the spring of 1568 and was imprisoned in England from then until her death. From then on, the story is fairly boring until her final trial in the fall of 1586 and execution in early 1587.

Shame history was not like the way the show is written.