I believe some math is in order: (post is long so I am sorry in advance)
Series rank 25 requires an individual to acquire a total of 108,000 Series Rank EXP
Frontlines, being a mode where there are 3 different outcomes, is a bit difficult to average per say, but generally speaking if you were to get a perfectly even amount of games where you come in 1st, 2nd, and 3rd an equal number of times, this would be equate out to roughly 108 games. FL games run, on average, 15 mins from my experience so this would roughly take, on average, 27 hours to complete assuming the average.
Crystalline Conflict on the other hand is a bit easier to math out correctly since there are two possible outcomes. Assuming the average, this would be 137 rounds of Crystalline Conflict. The shorter match timer does speed things up, but a single overtime where a stalemate occurs can easily cause the match to run as long as two or three matches since overtime's time limit is theoretically infinite. As such, the average time used will be equal to the duration of a match, being 5 mins. Using this metric, the average time required for CC is about 11.4~ hours total.
Note none of the calculations consider queue timers as well as the time taken before each match where you are stuck in spawn. Admittedly, while I could math out Rival Wings, the issue here is that there is a finite amount of time per day where it is possible to queue RW since it does not have a daily attached to it and it is not the fastest method to acquire series rank, meaning unless you are keen on DC travel, this method will naturally be the longest due to the active window for matches being lowest.
Finally, it should be noted that PvP in its current state is extremely unbalanced. It is not quite like PvE content where (to an extent) every job is viable in the content to a degree. Solo queuing into PvP will generally show that there are jobs that have a natural advantage for generating wins for you and your team, provided you know what you are doing and you team knows how to capitalize on what you are doing. As such, there are also jobs that tend to favor queuing with someone else due to the level of team play those jobs need to be fully effective, which for CC is an issue since you can only solo queue into this format. As such, even by quantifying how long it would take to do said grind, there is a high chance that your job is not cut out for particular PvP modes due to the balancing, resulting in more losses than wins.
tl;dr you can do it in a day via CC but you will hate yourself at the end of it due to imbalanced and unfun game design