Gap between successive visits in a long term visa

[pulimiharish]

After getting the 6 months visa for 3 times in succession my parents have got 2 year long term visa in Last year July (July 2015 to June 2017). They have visited once (July 2015 to Dec 2015) and now I want them to visit us in the summer this year (May 2016 to October 2016). But the gap between these two visits would be less than 6 months. Are they fine in visiting us with a gap of 4 months or is it too risky and suggest to visit in the same time frame (July 2016 to Dec 2016)?
Thanks in advance

[Casa]

It's risky. They shouldn't be spending more time in the UK in a rolling 12 month period than they are spending in their home country.

[secret.simon]

This topic is getting slighty too repetitive in this forum. Would the mods consider a Sticky on optimal visit visa periods and similar topics?

To the OP, also see other similar threads in the post below;
http://www.immigrationboards.com/genera ... l#p1285564

[pulimiharish]

Thank you for the replies. I thought they can visit us in the summer to avoid the winter but as its too risky I'll have to change the plan.

Thanks again