Training time could be increased if you remove AWs that speed up the production for specific barracks, or add armories or level up AWs that increase training squad size.
Ah, forgot - there are also expiring Garden Stones that have same effect as Shrooms.
I have more than 28 hours for all 5 slots, and have no problems with army.
If you need, you can set up smaller training squads in browser using slider at the bottom.