If by "first training stage" you mean Recruit Training, only a single approximately 2 hour long class is required to sort out mods and conduct an introduction to the 506th IR RU.
After Recruit Training is completed properly, you attend AIT. In AIT you learn necessary in game skills, combat procedures and unit SOPs. AIT is 4x two hour long classes that span a minimum 2 week period. During this time you may shift from one instructor to another if absolutely necessary but it's really preferred that you not. Should you have a need to change classes, inform the instructor of your previous class so he knows not to expect you, as well as your new instructor so that he knows to expect you.
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