On the right side of your gaming screen, you will find 5 golden badges that will indicate how many in-game days your recruits need to finish their training. The less time your recruits need to train, the faster you get more soldiers on your counter. 

Upgrade and max out all the items in your military camp to earn new badges, and reduce the training time.

Take in mind that the three last golden badges will be unlocked thanks to the Seargent, the War Simulator and the Weapons' Workshop, which should be paid by gems.
Please also take in mind that objects in Extra Barracks won't affect the golden badges' progress since they also need to be unlocked with gems.