12 hours is actually on the low side for 4 piece- they need to train bars beam floor and vault, plus conditioning, plus preps and drills, plus dance...
So if she's doing 4 x 3 hour sessions, the structure will usually be warm up and basic preps for an hour, 2x 30 min apparatus work, an hour conditioning, preps and stretching. So they only get an hour a week on each of the apparatus.
It should be enough for local comps and club level, although it will depend on which region you're in. If it is too much, many clubs have squads which only compete floor and vault, which will be half the hours.
For the ones that love it, they never regret the time in the gym. If your dd wants to give it a try, and you can afford the time and money, do it.
Also, senior squad may not be "senior" as you would think. If she's 9 next year it may simply because womens gymnastics is done by age, so the junior squad may be the under 8's who are too young to compete, senior is over 8's who will be training for national grades.
When you have your meeting, ask!