When to enter the UK

Hey all,

I've had a search of the forums, as well as the internet and the UKBA website. What I'm trying to figure out is when is the latest date my wife and son can come join me in the UK? Our spouse visa has an issue and valid from date of April 3rd, from my research it looks like they have to enter the UK within 3 months, to be eligible for ILR in 2 years (27 month visa - 3 months)

Does doing this (waiting until July to come over) have an impact on our ILR application in 2 years?

The reason we are even considering it all, is because I am going back to the US for a vacation in June, so we didn't want to spend the air fare here for us all to go back in a month.

Thanks in advance for your answers.