Consecutive visits of parents

[APG123]

Hi,
My parents were visiting my brother and me in the US for about 6 months (30th Sept 2012- 27th Mar 2013). I would also like for them to come back in the US again for another 5 to 6 months from August 2013. Is there any rule which states that they cannot stay in the US for more than 6 months in a calendar year? Both my parents are retired and this would be their 3rd visit to the US (since their visa was granted in 2008); first in 2008(~3 months), then last year and again this August. They have a 10 year multiple entry visitors visa. I would really appreciate your help and guidance in this regard.
Thanks!

[peace999]

There is no set time rule.

It is all at the discretion of entry officer . If he thinks you are staying long in US rather than visiting, he has full authority to cancel your visa and send you back. In reality, for parents in such a situation, they interrogate them . And let them in for a shorter stay with a warning for the future.

For your parents, chances are they will be let in in august but they could get a short stay after questioning.
Defer their visit by another 2 months to avoid any potential hassles at POE.

Also remember that if they will renew their visa in 5 years time, their history of long , frequent stays and extensions
will be taken into account.

[APG123]

Thank you so much for the quick response!