I know that Suzune has been a Magical Girl for a long time (considering she's 18 by the time of Suzune Magica), but I can't seem to find any source for how long she's been a Magical Girl assassin.
I doubt she's been under Kagari's control for 5 years, considering Kagari is Matsuri's twin and is supposed to be around the same age as her. With the fact that Matsuri is supposed to be in middle school by the time Suzune Magica starts, and the wiki estimates her age to be around 13-14, that would meant that Kagari would have to be 8 or 9 by the time that she started her revenge scheme. I can understand if the plan was anything from quite a few months to a little bit more to a year at most, but 5 years is way too improbable, unless Kagari edited Matsuri's memories and she's much older than we think she is.