Dear Squeenix: Instead of using FOMO to promote PVP...

[anw]

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.

This is about right, my own calculations made for somewhere else came out at 118,000 S EXP but it could be 108k. I calculated for 118 and came out with 148 games and used an average time of 6 minutes per game (accounts for setup & post game) flat and looking at adding in the queue times, overtime etc separately. It works out with setup to be around 13.5 > 14 hours based on your 137 games figure and almost 15 hours on 148.
As you said there was no time taken for queue timers etc, so if we give an extremely swift & generous queue time of 45 seconds we're looking at a little over an hour but there's going to be natural downtime in there.
Time to dump tomes, dump wolf marks, stretch, tend to your body and accept life exists. For a grind that's around 14-15 hours you're looking at something more like somewhere between 2-5 hours of downtime.
Overall it works out as a little under a day but definitely in the range of 15-18 hours when you account for variables and downtime.

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

You certainly could, but I don't think any health professional would recommend even your lower 11.4 hours estimate in one day. We only have 24 hours in a day, if someone is extremely efficient and sleeps for 4 cycles lasting 6 hours & 15 minutes either way for waking/sleeping then you have 17.5 hours to get your XIV PVP wins in. That's not a lot of time for health, the downtime or queueing.
Players learned during this season that grinding CC isn't so easy as knocking out a swift 15 hour session and that leads us onto the next point.

[ZavosEsperian]

You certainly could, but I don't think any health professional would recommend even your lower 11.4 hours estimate in one day. We only have 24 hours in a day, if someone is extremely efficient and sleeps for 4 cycles lasting 6 hours & 15 minutes either way for waking/sleeping then you have 17.5 hours to get your XIV PVP wins in. That's not a lot of time for health, the downtime or queueing.
Players learned during this season that grinding CC isn't so easy as knocking out a swift 15 hour session and that leads us onto the next point.

Admittedly, the character limit got the better of me because I had to cut a portion of my explanation short to keep it inside the limit. The times I listed were mainly related to overall match times and not all of the extra time involving queues and the 1 minute of setup time. It would likely come out closer to the time you gave if adjusted for that but even then it is somewhat variable where the time can be as short as 15 hours but the upper bound limit can be extremely high for some players due to when they can play the game not lining up with primetime.

No sane person would, or should, grind everything out in a day for a multitude of reasons however people will likely do it because of the rewards since they, until an alternative method is given to acquire them, are affected by FOMO. This type of content does not fit in with the rest of the game's content philosophy in my view.