How long before the date of travel is it best to book a last minute holiday?

We are going to Tenerife on 3rd July and now my brother in law and his girlfriend have decided that they want to go around the same time.

I have told them that I will keep an eye out for them on the various holiday company websites but can't find anything for them.

They can travel anytime from 1st July onwards and don't want to spend more than £850 for 10/11 nights.

From what I can find they will be better with Self Catering and I have told them the money they can save with this they will have extra to eat where they want anyway but they won't listen.

Any help would be appreciated.

Many thanks