Honestly, at five, I think 30 minutes every other week is fine to start with. You won't see much progress- but you may not see much progress at that age anyway. It's more about building confidence on a horse as well as getting some idea about the skills. It obviously depends on the five year old, but in my experience of teaching children, many don't have the balance/co-ordination to progress quickly until they are about 7/8- and a whole hour on a horse is pretty tiring for them too when they are very small!
If she looks to be doing well, it may be better to start investing in private lessons then.
She may not be allowed to help with horse care until she is a bit older, but if the stables offer this, it is good!
Others have given you a good list of things to look for but on my list would also be:
-Have they got enough ponies of appropriate size and temperament for everyone in the group. It's not helpful for small riders to be "overhorsed", and can damage their hips if the pony is too wide.
-Are the stables generally well organised, well maintained and well kept?
-Is all the equipment well organised and well maintained?
-If you are going to be borrowing a hat, do they have one in a small enough size for your DD?
-Do they offer boots to borrow, or else have cages on the stirrups to stop feet slipping through?
-BHS/ABRS etc is good and guarantees minimum standards of welfare and safety.
-Are they able to offer hacks/trail rides? (this isn't essential, but is very much a nice to have in my opinion).
-And most of all, do the riders in the lesson look like they are having fun in a safe environment?
If you are able to go to the stables at a quieter time, then someone will almost definitely be able to have a chat about what they think the best thing to do is, and show you around. A good riding school knows that supporting "non-horsey" parents is part of the job!