Why can my brother be asked to serve 29 out of 36 months?

[sicksicksick_N_tired]

He's a former Marine and presently a National Guardsman.

Are there no limits?

[Peregrine]

1. Not clear what you are asking

If he was enlisted he actually has a 6 year obligation. He can be called back to active duty to serve up to the six years. They have activated 50 year olds because they have a specialty that is needed.