I think 15mins. is far from being enough. It's toooooo less. You won't be able to change your whole 11 and probably 1 or a few of them will get injured (up to their condition). In 24hrs , if there will be 2 matches, there should be 9-12 hrs time difference btw these 2 matches. Players get 5% condition in every 3 hrs. So, if the time difference extended to 9-12hrs , players will have a chance of getting 15%-20% condition.
With current system you have to have lots of green boosters everytime. If you are out of green-boosters, you will need to buy it or get it as gift from friends. Otherwise, your players will get injured and one by one and at the end you will need to buy red-boosters, unless you can not get it as gift from friends.

By the way, I have a question. My many friends send me gifts red,blue,green booster or training points. For example, 5 of my friends send me red-booster but I only get 1 in total. Is it a bug or this is how it works ?